A full wave loop antennas impedance and choosing the correct Choke/Balun

Recently I've discussed with some of my customers what is the correct BALUN or common mode choke to use for their proposed antenna system. Specifically full resonant full wave loops.

In this blog I wanted to go over some basics of resonant loops to ensure you select the correct product for your quad loop. I'll also try to explain some of the matching requirements for loops in general.

One thing you will need to be aware of is the shape of your loop antenna will change its impedance. Also, a loop if typically, a rectangle or triangle in shape. Here are a few example loops.

A triangle loop

A diamond loop

A rectangle loop

The circle on each of the loops represents the feed point of the antenna, the colours Green, Blue and Red represent 200ohm, 112 ohm and 50 ohm feed points at resonance.

A good match for modern transceivers is 50ohm, therefore it is possible to design a loop for a 50ohm match. We're not going to do this.

We going to model a 200ohm loop for 20m in MMANA. This will show you the feed impedance of the antenna and what SWR it will present to your transceiver if the impedance is not matched correctly.

Firstly, our model, this is a quick 4 wire model and is not perfect, bit long, but it will do.

If we make this antenna, it will have the top wires 7.71m long and the sides 4m long, the sides being nearly 1/6th of a wavelength. It's twice as long at the top and bottom than each side.

We are assuming the transceiver impedance in MMANA is set to 50ohm's and run a calculation for the antenna. I have assumed the antenna is quite high 20m, with the bottom being at 16m.

This is what our calculation shows.

Loop calculated at 50ohm match

You can see highlighted in blue the R (ohm) resistance at the feed point for 14.150 is 222.3 ohms calculated. If you attach this loop to your transceiver, you will see an SWR of 4.48. The antenna is a little long, if we look at an SWR speculation in MMANA we can see this quite clearly.

SWR speculation showing the antenna is too long

The feed impedance is set to 50ohm just to show you there's really nothing you can for you SWR with this shape antenna expecting a 50ohm feed point. The SWR will be about 4+. So the choices are a) use and ATU of some form or b) change the impedance at the feed point (change the antenna shape and dimensions) or c) match the feed point to the antenna.

To show a basic feed point match in MMANA I will change the expected feed impedance to 200ohm and run the model again.

Calculation assuming 200ohm is needed at the feed point

You can see the R (ohms) has not change, however the SWR has. We can now re-run the same SWR speculation, assuming we require a 200ohm feed.

SWR speculation antenna is too long, but we have asked for 200ohm feed impedance

We now have a good match across the 20m band, under 1.25. The impedance in ohms of the antenna has not changed. What we have done is use a 4:1 matching unit, like the radio stuff 4:1 Balun.

What has happened is the 222.3ohm impedance of our model has been divided by 4 but the transformer, hance 4:1. or, 222.3/4 = 55.57ohms which is close to a 1:1 SWR.

So when you put up a loop try to consider what it's going to look like, and if possible, model it. For the 50ohm, 112ohm and 200ohm feed points we can provide 1:1, 2:1 and 4:1 balun.

Just for fun I'm going to try to tune the loop to the 10m band at 28.5mhz below.

28.5mhz using a 20m band loop

Awesome it also works on the 10m band (2 wavelengths), on 15m it will not present a good match (SWR about 12).

I've tried to keep this blog as simple as I can. Loops are excellent antennas, give them a try them and use the correct matching unit or choke and you won't be disappointed with the performance of the antenna.

M0VUE