13 - Experimenting with Chokes - The theory behind good ones!

Experimenting with Chokes 

<< changed 10th June 2020 to fix bad Al value for type 31 - redid graphs and tables>>

A real antenna is never symmetrical, it is affected by nearby wiring, pipework and pieces of metal or even conductive earth. If you use coaxial cable then there are three paths for current. Through the centre conductor, through the inside of the coax sheath and the outside of the coax sheath. The two surfaces of the sheath are independent of each other. Electricity at radio frequencies only travels on the skin or surface of conductors – this is known as the skin effect. (at 3.5MHz 2/3rds of the current is within a 1/30^th of a millimetre of the surface). We don’t want current on the outside of our coax for two reasons. It makes it act as an aerial – both for transmit and for receive and it brings RF into your shack.

When you unintentionally transmit via your antenna feeder it may reach nearby electronics and cause interference – obvious in the old days because analogue televisions were so susceptible to it (TVI) but nowadays the effect is much more subtle, burglar alarms may go off in the next street, your coffee machine may decide to switch itself on or it may even go unnoticed - your Wifi may drop to a snails pace or your phone may drop from 3G to 2G. Even if unnoticed you should try and stop it.

On receive, interference from your home, and the homes on either side of you may be coupled onto the outside sheath and these end up at the input terminals of your receiver. Either by going up to the antenna and back down the centre conductor or by lifting the earth potential of your antenna socket at the back of your transceiver. You may not notice this except to comment that you appear to have a noisy antenna or you may hear harsh buzzing or whistles or bursts of noise at specific areas of your tuning dial. Some internet lines coming into the house are inherently noisy at HF (VDSL in particular) and modern Wifi extenders that use the mains wiring, a lot of LED lighting and even smart phone and tablet chargers can cause problems (modern phone chargers use switching technology and may or may not have anti-interference components fitted – a lot of Chinese units fit the components to the units that are sent to get tested for EMC and then the production line omits the components once mass production starts. 99 times out of a 100 they get away with it.)

RF in the shack may or may not get noticed – this year, but maybe you change something next year and strange things happen – your transceiver won’t switch from transmit to receive, your internet goes slow (I had this problem and only noticed it when the spectrum waterfall on websdr.org froze when I transmitted at 100W on 17m, it was ok at 50W and I fixed it with a choke) or your lips get burnt when you kiss your microphone (don’t kiss your microphone!)

The current travelling down the outside of the coax sheath can be reduced to nearly zero if you insert a choke or two. Usually called a common mode choke(CMC) as it does not affect the differential current that flows up the centre conductor and back down the inside of the coax sheath.

If you wind coaxial cable in a coil then the differential currents don’t realise this as they are effectively shielded from the outside by the thickness of the sheath but the outside of the sheath notices as it is now having to pass through a coil – a coil that has some inductance and hence reactance measured in Ohms. A 500 Ohm choke in a 50 Ohm system will reduce the common mode currents to 10% of what they were – current from antenna asymmetry elects to take the path of least resistance (sic) and travels down the inside of the coax instead (it acts as a balun but I don’t like the term balun or unun – I try to use the terms chokes and transformers).

If the current is induced onto the sheath from an external source then the resistance causes the interference to dissipate as heat – it doesn’t reach the shack (or the antenna)

However a coil of coax is only effective at one frequency, it possesses capacitance between its turns as well as inductance and this causes a resonance – which is good at some frequencies but poor at others. Resonance can give unpredictable results and is difficult to “tune” to the correct frequency. The coil must not hang near a metal mast either.

For multiband performance, chokes should use ferrite material such as toroidal rings or tubes. Ferrite beads are sometimes used but they are very expensive and heavy if they are to be effective. I will concentrate on ferrite toroids and not powdered iron toroids. Powdered Iron should not be used as chokes, they (a) won’t have enough inductance and (b) are low loss (high Q) and we want lots of loss in a choke. High Q circuits make poor chokes as the resonance peaks, whilst of a very useful high impedance is only present over a narrow band of frequencies and that frequency moves according to stray capacitance. Powdered Iron make good filter inductors and narrow band transformers. Powdered Iron toroids are named with a T such as T200-6 and they are nearly always painted, red, yellow or glossy black etc.,

We can use ferrites to make broadband transformers, where different criteria apply. I am talking about chokes here.

Ferrite Toroids are named with an FT prefix, such as FT240-43, this has an outside diameter of 2.4” and is of type 43. Other common types include -31, -52 and -61 as well as several in the 70’s which are better suited to switching power supplies and frequencies of a few hundred kilohertz. You occasionally see them used on top band (1.8MHz).

Manufacturers produce datasheets that can be confusing. They sometimes quote that a given ferrite type is suitable for a certain frequency range and then you see them being used at other frequencies. The reason is that a ferrite can be used to build a transformer where we want low loss, or a choke where we want high loss. Also sometimes we want resonance and sometimes we don’t – or we want a wide band smeared resonance and not one that is too peaky. A narrow peaked resonance does provide very high attenuation but external reactances will detune and move the peak.

Also type #43 is really common and widely available – it is good enough even if used on non-optimal frequencies. The more recent #31 is a better choke at lower frequencies (say below 10MHz) and very nearly as good at higher frequencies. Of course choking impedance is not the only design criteria – we should worry about power handling and how smooth the response is over the entire frequency range of interest.

I found the excellent charts produced by Steve Hunt G3TXQ(SK) to be a good starting point – he did a lot of choke measurements. My studies took me to excellent documents by Jim Brown(K9YC) and some really useful calculators and spreadsheets by Owen Duffy(VK1OD). You should study their websites for further information.

Here are diagrams found on Steve’s web site http://www.karinya.net/g3txq/chokes/

Here we see that 17 turns on a single FT240-43 should give 4k to 8k of choking from topband to low down in the 10m band. But it is only resistive from 4.5 MHz to 12 MHz (the thin black line shows this, but we really need to know values)

On the face of it the type #31 seems not as good as the #43, 12 turns on a single

core FT240-31 gives 2k to 4k of choking over all bands. More subtly, and more importantly is the black line – the choke appears as a resistance of a large portion of the spectrum – the #31 is very broadly resonant and less prone to a peaky resonance, this makes it better behaved if presented with a nasty range of antenna reactances. In fact after further study I now do not use the charts above as we really need to know the resistive values (the details of the black lines above). The reactance, and overall impedance are less important, since you may be unlucky enough to combine a high choking reactance with an antenna which has the same reactance but of the opposite sign. In which case the choke does nothing. You really need resistance in a choke. I give my own graphs and tables of resistance at the end of this article. I will add them to my blog after this article is published.

Many hams have used the charts above and ended up with 12 turns of coax wound on an FT240-43 – a useful choke, certainly from 80m to 10m. However a closer inspection of the datasheets and the writings of others show that the relatively new type 31 (promoted since 2006) is actually better below 10MHz, and nearly as good from 10 to 30MHz. Again, for well behaved antennas either will do. For antennas well off resonance and presenting reactive loads then the 31 may be better.

Manufacturers do not really produce products to suit radio ham bands – they are in the business of producing ferrites for EMI suppression and also for transformers and inductances for filters. Fair-Rite make ferrites and Amidon distribute them. Also worth knowing is that for some reason Amidon and others rebadge the ferrites so what Amidon calls an FT240-31 has a Fair-Rite part number of 5931003801. Here are what the sellers of these toroids actually recommend.

¹Amidon(the distributor) is quoting a different range from Fair-Rite the manufacturer

² These figures are from Palomar Engineering, rest from Fair-rite and Amidon sites

 
Not a bit of wonder we can have variable performance from 3 to 30MHz! A lot of what is currently done is because originally we only had type #43 – it was easy to get. So custom and practice was to use #43 for 3MHz to 30MHz. It would be better to use type #31 for chokes from 1 to 10MHz and #43 for chokes from 10MHz to 30MHz. If you are buying fresh pick #31. Jim Brown says “a single #31 choke is good over a 8:1 frequency range while a #43 choke is good over a 4 to 1 frequency range”.

Here is another chart from Fair-Rite showing its recommendations for chokes

For ordinary inductors or transformers different rules apply, sometimes we pick material for a transformer based on its complex (sic) magnetic properties rather than its recommended frequency range; this is because it is hard to make transformers with a very wide bandwidth and choosing the number of turns and tweaking the transformers becomes an art not a science. This article is only about chokes.

Jim Brown (K9YC) has an excellent set of documents describing choke design – some of it relating to choking RF out of audio cables but he also covers RF choke design fairly rigorously. He sometimes suggest using 5 toroids at a time – which is a bit expensive, Ian GM3SEK has produced designs that are cheaper to make using type 31 tubes rather than toroids. I have FT240 toroids so that is what I will use. I will buy some of the tubes that Ian recommends (in http://www.ifwtech.co.uk/g3sek/in-prac/) as well as some clip on ferrites the next time I go shopping for Ferrites.

Toroids are given values in their datasheet for Al and ui – the Al constant is used for a quick and dirty calculation of inductance. However this is of limited use with ferrites as the value is only accurate at very low frequencies. Real ferrites have a reactance that may start out inductive but at some stage it actually becomes capacitive! And it has considerable resistance as we will see later. The initial permeability ui is just that; an initial value measured at a unrealistically low frequency– real ferrites have a permeability that varies with frequency, and it is not even just a simple number, it is best considered as a complex number, it has two parts µ’ and the µ’’.

The datasheets for the Ferrites show the µ’ and the µ’’ values – these represent the reactive and resistive (losses) values if we are making chokes. They actually chart the complex permeability but this will approximate to reactance (µ’) and resistance (µ’’) – related to the losses in the material. Losses are good in a choke 

So we can anticipate the resistive portions being quite low up to 3MHz and dropping again after 30MHz For comparison here is the #31 graph

Note these have slightly different scales. – at 2MHz the type 31 has 4 times better resistance choking.

I wanted chokes specifically for a new End Fed Half Wave antenna which covers all bands from 80 to 10m but needs good common mode chokes. I resolved to design something with as much resistance as possible. Current thinking (since 2006) has been to try for 5,000 Ohms. It used to be thought 500 Ohms was enough, to choke off the more obvious RF currents. However 5000 Ohms helps reduce receiver noise.

Rather than use Steve’s charts I wanted to calculate my own. I needed tables of µ’ and the µ’’ to build up my spreadsheet. There are tables on the Fair-Rite website at 1MHz intervals which I imported into Excel’s polynominal curvefitting. This allowed me to get a table for the hambands. I then found that Owen Duffy had beat me to it and used more sophisticated cubic spline curve fitting, he had also re-measured some of the values so I decided to use his µ’ and the µ’’ values. Thanks Owen.

The formula for R and X depends on the initial ui and the µ’’ and µ’ values at the frequency (f) of interest. You also need the number of turns (N) and the published Al value for your toroid. Ferrite Al values are only given +/- 20% so the graphs and tables below are only roughly indicative. The formulas for Resistance and reactance are;

Rs = (µ'' /µ_i ) * 2πf N² A_l

Xs = (µ' /µ_i ) * 2πf N² A_l

There is one other wrinkle. Real chokes suffer from stray capacitance which alters these equations. I copied an idea of Owen Duffy’s website and re-calculated the resistances with a stray capacitance of 1.3pF, I will be experimenting with different values when I try measuring some chokes.

here are tables of µ’’ and µ’ – from https://www.owenduffy.net/calc/toroid4.htm

You can find the manufacturer’s data at

https://www.fair-rite.com/43-material-data-sheet/

and https://www.fair-rite.com/31-material-data-sheet/

(there is a link to a . CSV file halfway down the page )

Using the table of µ’’ and µ’ above, I built a spreadsheet and from it produced this graph and table. The spreadsheet on my blog calculates X and Z as well (but those values are irrelevant – Resistance is what matters, at least in demanding applications

Above is a table of resistance for differing turns of RG58 wound on a FT240-31 , I like the look of 15 turns, for 80m to 20m.

Here is the data for type 43;

Here 11 turns looks good 20m to 12m. To get even better performance I decided to add these two chokes in series. The final R,X and Z values for this combination is the R_TOTAL column shows the two chokes together exceed the 5k criteria for resistance (and nearly 10k for Z)

Below is a diagram of a choke

- this one has 8 turns including a crossover winding known as a Reisert Turn.

This allows the input and output to be on opposite sides of the choke and allows more turns. It may reduce capacitance slightly though this is very hard to measure. It seems to make no difference but is easier to wind

In Summary

Current thinking is that common mode chokes need high resistance, high impedance (reactance) is not a reliable way of choking badly behaving antennas.

A single choke will not cover all bands. An air-spaced choke may sort of work on one band.

A lot of commercially available chokes are old fashion designs and poor performers in certain situations. High Resistance chokes always perform well.

The tables and graphs above are new and should be useful to anyone wanting to make a common mode choke. – at least from FT240 types of ferrite. The spreadsheet on my blog can be used for smaller ferrites, although getting RG58 on them is tricky.

I will attempt to measure my chokes in a future article – or try my blog http://MI5AFL.Blogspot.com. Note, measuring high impedance chokes is actually very difficult – I have already fallen foul of my scope probe’s built in capacitance and had bother with heavyweight manipulations of S11 and S21 data in my VNA. (and getting bad data from the internet - http://toroids.info  has the wrong Al value for a FT240-31, http://amidoncorp.com has the right value.

My study of this area has depended on the work of others; in particular

• Owen Duffy VK1OD at http://owenduffy.net
• Jim Brown, K9YC (http://k9yc.com/publish.htm
• Steve Hunt, G3TXQ (SK) (http://www.karinya.net/g3txq/chokes/ )
• Steve wrote an excellent article for Radcom Plus in May 2015 – I wish I had read it early in my research!
• Chuck Counselman, W1HIS, (http://www.yccc.org/Articles/W1HIS/ )
• Ian White,GM3SEK (http://www.ifwtech.co.uk/g3sek/in-prac/ ) & also (https://gm3sek.com/ )
• Rick DJ0IP (https://www.dj0ip.de/rf-cmc-chokes/ )

If you go to the "pages" tab of this blog (http://mi5afl.blogspot.com/p/blog-page_15.html )you will get a link to various files I hae written  - the txt of this article as a pdf and three spreadsheets, Ask me if you want me to explain anything.
(mailto: MI5AFL@ARRL.Net )

12 - Measuring the impedance of Coax using a nanoVNA

Measuring the Impedance of Coaxial Cable 

Published in Contact - the magazine of the Bangor and District Amateur Radio Society (BDARs)

I have many random lengths of coaxial cable, some marked and some not. Whilst most of it is 50 Ohm cable, some of it is 75 Ohms and I needed to find out which is which.

I bought a NanoVNA for about £30. The screen is small but I only use it attached to a PC (NanoSaver software). I have used the NanoVNA as an antenna analyser but you can use it for so much more it can measure resistance and reactance at various frequencies.

There is a reasonably obscure way of working out the impedance of unknown cable by using the Lamda/8 method. Unknown to me until Google found a post in the NanoVNA group.io pages. It is very simple to do – attach a length of the coax to the main port of the VNA with the other end of the cable open circuited.

You sweep the cable for its R +/- jX impedance over a range of frequencies from low to high and find the frequency of its first ¼ wave resonance. This is easily seen as a ¼ wave of transmission line will transform the open circuit at the open end of the coax to a short circuit – zero or low resistance and zero or low reactance. It doesn’t matter what length of cable you use. The same data can be used to calculate VF, the Velocity Factor but that didn’t matter to me on this occasion – I just wanted to make some Chokes and Transformers.

Having found the frequency where R and X were lowest – note on the graph below that the scale for X has zero about half way up the right hand. I got 13.44MHz and an impedance of 2.284 – J1.18 Ohm (I happened to be using about 5m of cable so I knew 13-14MHz was in the right ballpark for a quarter wave)

The lamda/8 method requires measuring the impedance at half this frequency so I dutifully set the frequency to sweep from 6.5MHz to 6.8MHz and read off the impedance at 6.67MHz which is seen in the next diagram. – we need the reactance figure – the one after the – j in this case you see that this is Z = 48.4 – j 52.6 so this is “50 Ohm” cable (actually 52.6 Ohms).

You don’t need to know how this method works but if you want to know; it is because the impedance Z of an open circuit transmission line is given by the formula

Z = -j Z₀ COTANGENT ( 2πf * Line_Length )

Line_Length expressed in fractions of wavelength or fractions of cycles of Frequency.

If the line_length is lamda/8 then we have the COTANGENT (π/4) which is 1 so at that particular magic length the measured impedance is –j Z₀

There is more detail at http://www.antenna-theory.com/tutorial/txline/transmission6.php although it is fairly heavy going and it uses a slightly more longwinded approach of measuring impedance of the open circuit coax and the impedance of the short circuited coax and then using the formula.

Z_{0 =} √[ Zshort * Zopen ]

I tried this alternative method on my VNA and got poor results, (10 Ohms out) Although my AA-30 Rig expert antenna analyser gave readings of 48 Ohms which is reasonable. I think the Lamda/8 method is simpler to use, avoids square roots and complex numbers! A third method sometimes quoted is to use the square root of the shorted cable inductance divided by the open circuit capacitance. My VNA gave inaccurate results using this. As did my AA-30.

 

While writing this I found a very good reference at http://www.rfcec.com/ in chapter 3 part 27 – measuring Zo. He says the lamda/8 is more accurate using a shorted cable (and a modified equation) but it is ok to use the open circuit cable – my results were fine.

Now I know I have 50 Ohm cable I can wind some chokes on my FT240 Ferrite cores!

Next article is on how to design chokes

11 - Results from my OCFD (SWR) - article written for CONTACT

Submitted to BDARS for inclusion in the monthly magazine "CONTACT"

My Offset Centre Fed Dipole

I finally got to tune the OCFD I talked about at the last club meeting. The antenna is suspended between three poles from 7m to 10m high.  My experimental work with MMANA-GAL had given me lengths of 8.3m and 12.6m – a Dipole with the 200 Ohm feedpoint at 39.6%. expected to give reasonable SWR but not perfect on 40,20,15 and 10m.

Above is a photograph of the necessary transformer and choke, all wound with FT240-43 cores. The pair of cores in the transformer are wired in series-parallel to give an overall turns ratio of 2:1 which means they act as an impedance transformer of 4:1, matching 200 Ohms to 50 Ohms, or anywhere from 400 Ohms to 100 Ohms if we accept an SWR of 2 to 1 or less.  MMANA produced predictions of the graph below. 

The SWR predictions for 40m is 2.6 although 20m and 15m are very good at 1.3 and 1.6 with the 10m band prediction drops to 1.7 but the bandwidth for swr <2 is only 600 kHz.

As for the measured plots, I took two sets with the wires at 8.3 and 12.6m using my Rigexpert AA-30 antenna analyser and my NanoVNA. Both figures agreed and showed the minimum SWR at 6.85 MHz. I wanted to move this to 7.1MHz and therefore had to shorten the antenna to 96.5% of its original length – I shortened the short side by 11.5 inches and the long side by 18 Inches and got the plots below.

Here is the overall plot – impressive how similar to the MMANA predictions, nice that the SWR is actually lower, particularly on 7MHz.  I now need to get WSPR going to see how well it actually works

Better than a simple dipole!

10 - Antennas: Experimenting with MMANA - Dipoles and my OCFD Offset Centre Fed Dipole.

Having been inspired by M0MCX, Callum's youtube videos on MMANA-GAL I decided to give it a try. Callum owns DX Commander and makes many videos - his DX Commander Vertical antennas look very interesting, if I had room for the radials I would build one for the shack. (You want a 30 foot circle for his 40m and up vertical made from a fishing pole.

MMANA-GAL is a free package for modelling antennas, I experimented with a simple dipole - at several heights, with drooping ends, with a run of wire near one leg to see what effect it had.

I give most of the details in the previous post - an article I wrote for BDARS's magazine "CONTACT"

After playing with 40m dipoles at 7m and 9m I moved on to consider Off Centre Fed Dipoles;

These are traditionally feed at the 33% point along the wire instead of the 50% in a simple dipole. At 33% you get a feedpoint impedance of 200 Ohms and meed a 4 to 1 transformer to match to 50 Ohm coax cable feeder. You also have some asymmetry and hence RF flowing down the outside of the cable; You need a choke to reduce this to zero.

I researched OCFDs at length, and was very much impressed with the website of Rick DJ0IP (https://www.dj0ip.de/ ). and also the groups.io page on OCFD (https://groups.io/g/ocfd )

I also used excel to plot some diagrams to see how other % splits would match - and then used MMANA to see what SWR was achievable (with a 4:1 transformer and choke)

The various sinewaves and parts of a sinewaves are plotted assuming a start position on the left hand side - in practice it doesn't matter if you start with the opposite polarity as the sinewaves reverse every cycle anyway - so  I show two plots below which makes it clear how different bands can be matched on the same antenna - if the currents (and volatges) are at a similar point the different waveforms can be matched. The ratio of current to voltage tells you what impedance you match to.

On the graph above you see that the traditional 33% gives superb matching on three bands, I wanted 4 bands so choose 41% (actually 39.7% after playing with MMANA) to get a compromise solution. The previous diagram shows how 14 to 19% would also give 4 bands but I was afraid that the gross asymmetry of a 14% antenna would create RF in the shack, and RF on the coax could (a) create interference locally - my name would be mud if my rf switched on the coffee machine in the house! and (b) pick up local interference and reduce my receive performance.

After much experimentation with MMANA I made an OCFD with legs of 8.3m and 12.6m but after measuring SWR (with a RigExpert AA-30 and a nanoVNA) I reduced the short leg by 11.5 inches and the long leg by 18". This was to move the SWR minimum point from 6.85MHz to 7.1MHz (a reduction to 96.5% of initial length)

The final result was a four band antenna that didn't need an ATU - apart from extremities of the 10m band. I could fiddle a bit more but I actually have an ATU inline with my transmitters - an MFJ-969 and I use the SWR meter and dummy load. Adding a tiny bit of loading will not waste Tx Power so I am now setup and will use it as is.

I have reported the tuning performance in another post here - the contects of another article I wrote for CONTACT, for my local club BDARS.

Here is my combined Transformer and choke; I now know that I didn't need to wrap tape around the FT240-43 cores but its hard to take off! All cores FT240-43. I include some downloaded schematics from the web that should help you understand the manufacture of this. I drilled a couple of 1/16" holes in the bottom of the box to let heat and condensation out.

(8 turns - I used more)

Next stage is to setup WSPR and see how it gets out - I have worked a couple of stations and it seems to receive well enough.

My next project is to get on 80m I hope to use an End Fed Half Wave (EFHW) that's 132 feet long - see the appropriate post for that when it gets posted!

Three masts the centre one supports the blue box.

9 - Using MMANA-GAL – Free Antenna modelling software

Using MMANA-GAL – Free Antenna modelling software

(Copy of article given to Bangor and District Amateur Radio society for their members magazine "CONTACT" )

Antenna modelling software can be complex but you can use it in a very simple way to get useful results. The free package MMANA-GAL is well supported with Youtube videos listed at the end of this article but in practice you only need 4 or 5 commands to use it effectively.  This article makes more sense if you download and install the software and run it whilst reading!

As an example of its utility, what if you were thinking of putting up a dipole for 40m, what's the difference between having it at 7m or 9m. What if it was made of thicker wire. What happens if you droop the ends to make an inverted V? You can "test" all this in software. You can specify the metal the antenna is made of (e.g Copper or Aluminium) and what thickness. You cannot specify a velocity factor in MMANA which means insulated wire ends up a bit longer (95-97%) but that is no real hardship. You can specify different types of earth, although I have only ever used the default "normal earth".  The results are useful for comparison purposes, don’t assume they are 100% right but they are a reasonable estimate of the truth.

Of course once you start fiddling (experimenting) with MMANA you might not find time to actually make antennas - so you do need a bit of discipline to avoid wasting time (having fun).

To use MMANA quickly note that the opening screen has 4 main tabs - you only need the "Geometry", "Calculate" and "Far Field Plots" tab. You also need to access the wire editor

To draw a dipole you need to run the "Wire edit" tool using the 6th icon in from the right or hit Control-W.  You also need to know that the view you will see has XYZ directions where Z is the height. You can think of X as pointing East and Y pointing North as the default directions. Once in the wire edit window click on the button marked XY. The diagram below is a screen dump of the Wire edit window.

The wire edit window has an icon tool for editing a wire and another one for creating a wire. These are the only two you need. You could instead enter all data into a table - I usually create a simple wire model, with approximate lengths, then nip into the table and adjust the lengths to what I want. Depends how good you are with a mouse!

A couple of tricks; you can specify height, either when you draw the antenna, or later when you go to calculate its performance. I usually leave it at zero when drawing.

Example: A dipole, one wire 20 m long and fed in the middle; in the Wire edit window, (XY view) draw one line that crosses the corner where the blue Y and X axes cross. It should turn red. Click OK which takes you back to the main geometry window, listed below

Edit the X1 and X2 values above to the lengths you want and click on the box marked PULSE under the Sources 1 window (bottom LHS) and enter the text w1c – this puts the source at the centre of wire 1

Switch to the calculate tab and Set 4 things; the frequency, the type of ground (“Real”), the height and the material of the wires e.g copper. You are then ready to click the “Start” button at the bottom LHS. Your window should look like that below;

Don’t worry about the SWR if it is below 2. – in real life it varies with your type of earth and antenna height.  You can also look at where the best directions are and the relative signal strengths at various angles. Click “Far Field Plots” up at the top, beside the Calculate tab. (if you click on the “Plots” button at the bottom you get a static display.  If you click your mouse on the antenna plots you can get various gain values in different directions. The left hand plot is a bird’s eye view – but you are best to specify a particular angle of elevation. Antennas send signals out at the take off angle to bounce off the Ionosphere – google TOA MUF Calculator for a useful tool. (I found mine at http://www.kolumbus.fi/pekka.ketonen/TOA MUF calculator.xls ) Energy leaving your antenna at 5 degrees will go 2000 km (DX!), an elevation of 15 degrees will give ranges of 1000 km (Inter-Europe) and 30 degrees for 500 km for UK use.

 – click the “Elevation” button at the bottom and set the value to 5 degrees.

So at 5 degrees you get a “gain” of -10.7dBi. At 15 degrees you get -1.6dBi and at 30 degrees you get 3.3 dBi.  When using dB (or dBi) you need to think in terms of ratios and factors, 3dB is a doubling, 6dB is a factor of 4 and 10 dB is a factor of 10. You add the dBs but multiply the factors. Anyway – for this article I just want to show the relative “gains” at different heights. At 7m this antenna puts 20 times more power into UK stations as DX ones. From -10.7 to +3.3 is 14dB – 13dB is (10dB +3 dB so 13dB is a factor of 20 to 1 (10:1 times 2:1)

If we go back to the calculate tab and change the height to 9m, click start and use the far field plots we can get the relative “gains” at various angles, and we can repeat this for 12m to get the table below.  

Elevation angle/likely distance reachable Height of antenna	5° (2000km) Best/Worst Directions	15° (1000km)	30° (500km)
7m (SWR=1.07)	-10.7/   -16.3	-1.6/   -9.7	3.3/   -3.8
9m (SWR=1.45)	-10.1/   -17.8	-1.2/   -10.9	3.5/   -4.3
12m (SWR=1.91)	-8.9/   -18.8	-0.1/   -11.7	4.2/   -4.4

Note I also show the relative gain in the X direction (the worst direction) to the right of the ‘/’

Note also the SWR changes as we are close to the ground – if your coax is poor quaility you will see better values in the shack (try https://www.qsl.net/co8tw/Coax_Calculator.htm to see how SWR changes.

The summary is, height matters! Unless your antenna is a vertical (which can pick up local noise). All antennas work and all real antennas are compromises.

I expect to demonstrate MMANA-Gal at the BDARs March talk. It comes with many, many examples of quite sophisticated antennas – you can have traps and feeders other than the default 50 Ohm coax. There are examples of cobwebs, hex beams and more.

Google for MMANA-GAL(Basic version) or use http://gal-ana.de/basicmm/en/

Good YouTube videos on MMANA are available from Callum, M0MCX – who makes the DX Commander Verticals – all his videos are good! – his channel is at https://www.youtube.com/user/m0mcx/videos and if you search for titles such as

“Part 1 - Idiot Guide to Antenna Modelling - Vertical and Dipole” or

“COBWEB Ham Radio Antenna 6 feet off the ground”

He also has much older videos in a series, for example

“MMANA Tutorial Part 1 M0MCX 20m dipoles”