Tuning a Dipole or EFHW antenna!!!, and other ramblings.

When we sell kits the biggest question we get after the kit is built is "how to tune the antenna". So, we have written this short blog to cover some examples of how EFHW and Dipole antennas work and how to tune them.

Along the way I'll put some drawings in the Blog and make some general comments about antennas. I'll also show an example of why an antenna should be isolated from your coax. And you measure your SWR and Resonance and try to use some terms which can be confusing, but I'll try to explain what they mean.

I'm going to use computer modelling software for the antennas just to show you what an antenna looks like and how it behaves.

Finally, if you want the short version of this article go to the end of it, the summary.

How long should my wire be for a Dipole or EFHW

To cut your wire length for your antenna you will need to decide what band you are aiming to work on. For a Dipole this is simple it will be a single band antenna. For a EFHW it can be a multiple band antenna. But the start place should be the same.

We are going to use the Metric system for calculations. I personally don't like mixing metric and imperial measurements. Oh, the joy of 40m band with 66 feet of wire.

The calculation you will need to use is 143/the frequency in metric, then divide the results by the type of antenna you’re building e.g. full wave, half waver and quarter wave etc. Easy right, well yes but there are a couple of other things to consider. Firstly, the calculation does not consider the velocity factor of the wire, secondly it does not consider how you are going to deploy the antenna. However, this is a good start. Here's a few examples.

The good news is, if you stick to the velocity factor of 1, then you will always be trimming the antenna, not trying to add to it. Unless you are building an inverted V, then the wire can sometimes need to be up to 5% longer.

Before we discuss how to tune, I want to look at the antenna and how it works.

Let’s design a dipole antenna for 7.1mhz for using the calculation above a half wave of wire is going to be 20.14 meters long. If we model the antenna in MMANA-GAL at 7m high, we get this. I've chosen 7m high because I'm in the UK and not everyone has a massive garden or handy trees, so let’s assume 7 meters in height.

This is what the antenna looks like when modelled and calculated.

In MMANA-GL you will see in the antenna design above and some numbers below.

This is telling me that Z 50.54 Ohm is a good match for 50 Ohm coax, the SWR is 1.05:1. j2.195 means `there some reactive element in the antenna, simple put is this is 0 the antenna is perfectly resonant, or resistive meaning all your power will go out through the antenna at the design frequency.

The positive j means the antenna is to long (inductive reactance), if it was -j the antenna would be too short (capacitive reactance). However, this antenna is a good match, and you will have no issues using this it. Therefore, to tune the antenna which is too long you will in the case of a Dipole trim and equal amount from both ends of the antenna. To make it longer you need to add wire. This is why using a velocity factor of 1 for your wire is a good idea, it's easier to remove wire than add it.

I want to point out one other thing here. When looking at the dipole the large bendy blue line represents where the current flows in the antenna system. This is where you hear the expression that the dipole is current fed. It's feed at the middle, which is the highest current point at resonance.

If we were to look at voltage on the antenna the voltage it would be near zero in the middle and high at the ends. The RSGB have a nice representation of that image on this link. Or, peek below. This mean if you are feeding a resonant Dipole form the centre its Current fed. If the antenna at resonance is fed from the end, its Voltage fed.

This is why people describe the End Fed Half Wave as a voltage fed antenna.

Before we move on, I want to try to explain a few things re-current and volts. Current and Volts are both present in any antenna system. At resonance both a Dipole and EFHW have high voltage at the ends. So please keep the ends away from anyone who does not want a nasty surprise.

Also, if your antenna is not at resonance, you will not be feeding it from the highest Current or Voltage point. This is less of an issue for a Dipole but can be a bigger issue for an EFHW. This is the main reason we induce common mode currents on the coax. So, try to get your antennas as close as possible to resonance for these types of antennas.

Lastly, both systems do not need a ground plane or ground radials to work.

Just for fun, I'm going to add a third wire to the antenna. This will represent coax. The coax is dropping down from the antenna and the running along the ground 25m. for the model, I've had to put the coax .05m above the ground.

You will see on the model that we now have some currents on the coax all the way back to my shack 25m away from the antenna. This could be an issue for you, especially if you start to run any power. This is why you should isolate your antenna from the coax, to radiate RF where you need it.

My match looks amazing though. Anyway, no dipole conversation should end without the subject of radiation patterns being covered.

So back to my 7m high dipole without the shack feed lines it radiates, something like this.

What is happening here is the antenna is radiating most of its RF energy broad side to the antenna. Just look at the wire and the RF is coming off the side and not the ends of the wire.

This is great, err not always, but it does mean we get gain, Harrah!

Gain is where we remove some of the RF energy from one part of the antenna and direct it to another part. Ideally where you want it. Loss is where the antenna is radiating less.

If you want to operate station each side of the Dipole where the gain is more it's great. A tip is to align the radiation pattern to where you want it.

If I raise my antenna to 14m above real ground the radiation pattern will look something like this.

So, there's a little more gain at the sides of the Dipole. But wait!!!!!

Dam, my reactance and SWR have changed. What I'm trying to point out is the antenna shape, height, and length will affect its performance.

SWR is important because it's likely you will kill your finals if your SWR is too high, we recommend keeping it below 1:1.5.

The point here is height can influence how the antenna performs as with your conditions. This is why we do not sell antenna with guaranteed no tune low SWR. Because we do not know where it is going to be setup and what the surroundings are. Having said that after about 1,000 antennas only 2 have not managed to be tuned at clients QTH.

The End Fed Half Wave

I'm going to take the same Dipole and feed it from the end. I'm going to use a counterpoise as well at .05l or wavelength. There are loads of musings if a counterpoise is needed or not. I like to think it’s needed, other don't. But hey we will get into that when I have good evidence for both cases.

Here we go. So, I've bought my 49:1 or 56:1 or 64:1 EFHW, broad band transformer, auto transformer, Fuchs circuit, or however you want to connect it to your coax.

If I calculate the SWR of the end fed I'm going to see this.

This is because I have my modelling software assuming I'm feeding the antenna at 50ohm's. Well I'm not, the actual feed I need is going to be somewhere between 1,800 to 3,600 ohms depending on how high the antenna is.

But let's keep this simple, I'm going to tell my software the feed is 2,500ohms.

I'll run the calculation again and.

Ok, so the antenna looks long. This is because it’s more difficult than just changing the feed impedance on the software model for an EFHW. You need to model the matching transformer correctly and all associated components. That's not easy.

But let’s look at the numbers the original model with 50ohm feed, the Z was 2491 divide that by 49 and you get 50.8ohm.

So, the 49:1 is an impedance transformer. It's making the antenna think its 50ohm feed is a 2500ohm feed. I'm not saying antennas think, but who knows.

The radiation pattern of the EFHW looks like this.

So, in my view an EFHW and a Dipole are just about the same, just fed differently.

The real difference between and Center Fed Dipole and Half Wave End Fed

Now let’s get into the fun stuff. Let’s assume you have tuned your antenna. A Current Fed Dipole, let me see what will happen to it if I select the 20m band and put some power into it.

SWR of 77 OMG! This is because the current in the middle at 20m is minimal. This is the modelled current distribution below, remember a Centre Fed Dipole is current fed.

Let's try the 15m band, 21.050 and see what happens.

The currents look high, so I should be ok right?

Err no, still a little high. Look, the dipole is a good mono band antenna if built well. It does need a feed point current choke and it's not a multi bander. But they are efficient, and easy to build.

Now were going to flip this on its head a little and look at the EFHW. The principle behind this antenna is to do with it being voltage fed. Let’s look at the harmonics below, they are Currents.

Credits to Wikipedia for this image above, please read this link. If you view the diagram as being a Current view, the Voltage on every harmonic will be high where the current is low. So, the Voltage is always high on the harmonics at the end of the waves.

This is why you can only use a dipole on high current odd harmonics (that’s if you’re lucky and use a tuner).

However, this is where the EFHW works well. Being voltage fed you can therefore use pretty much any harmonic of the fundamental, or lowest frequency. That is if you could build a broad band transformer with enough range to cover all the harmonic frequencies (you can’t).

But you can for HF which is what we are interested in.

So back to my 40m EFHW and now on 20m, these are the results from MMANA-GAL.

That's a heck of a lot better than an SWR of 77 with the dipole. What this antenna is doing is feeding from the end and taking advantage of the harmonics. There are a few things with the EFHW you need to be aware of.

Firstly, if I tune an EFHW to 7.000mhz I would expect it to be resonant on 14.000mhz right. Well, no, As the frequencies on the EFHW go higher the antenna acts as if it’s a little shorter, capacitive.

This is why we add a small inductor about 2m from the feed point, this is to pull back some of the capacitive element. This is done by tuning at the lowest band say 7.000mhz using an inductor and pulling this higher band resonance down a little.

This is an easy thing to do, you put 5 or 6 turns around a 25mm tube 2m from your antenna. You could use a capacitor to do the same, by tuning at the highest band you want to use and the adding a capacitor of a determined value to being the lower bands to resonance. The inductor is easier.

Also, remember 80m band, it's 80m to 75m, see below.

With the EFHW you will not get a 1:1.5 or better match across the 80m band, especially if you’re going to work harmonics on the higher HF bands. To mitigate the issue where you want to work DX on 80, then you can pop a capacitor in the middle of the antenna this will shorten the 80m ban and leave the rest of the antenna pretty much unaffected. So, with some fiddling you can change how 80m works.

In summary

In summary, if you are building a centre fed dipole, just calculate the length of the wire needed using a velocity factor of 1.

Dipole would be 2x1/4l lengths. Make sure both lengths are the same.

EFHW would be to cut 1/2l length, that’s a single length of wire.

Measure the antenna using something like a NanoVna or antenna analyser. These tools will tell you if the antenna is too long or too short. They both typically have the values show above which will help you decide what to do with the antenna.

If you do not have any of the tools above, some Transceivers have an accurate SWR meter.

This can be used if you see say a higher SWR on 7mhz than 7.2mhz which means your antenna is too short. But this can be difficult to do, but it is possible. A NanoVNA can cost as little as £50m it's a great investment. Or join a radio amateur club and borrow one.

If your antenna is to long you will shorten it.

For a dipole you will need to shorten both ends of the dipole equally.

For an EFHW you shorten one end, the end not at the transformer side.

Being a multiband antenna, we would always recommend tuning on the lowest band of operation, like 7.000mhz to get resonant on 20m, 15m and 10m. Also, a compensation coil may be needed as well.

Thanks, it. Any questions please ask.