Question on tuning Random Wire

After a couple of fails I managed to get try a Random Wire working yesterday - tried it on 40,20 and 10. But I wasn’t happy with the tuning and SWR on 20m. 40m was ok and 10m was super quick with a great match.

I am using a Xeigu x6100 which I have learned the hard way loves to become part of the antenna and doesn’t cope well if it does - always choke it. So my setup is:

  • Xeigu x6100 on 10w - Common mode choke - 9:1 unun - 84ft radiating element in inverted V setup ; 17ft Counterpoise on the ground (+/- 2 inches wire length, ignoring the wiring for the unun /choke box)

20m took an age to tune and was hovering with an SWR between 2-2.5, sometimes 3. The “bandwidth” was poor, and few KHz up or down the band and i needed to tune again.

What does the wise oracle of SOTA advise on what to expect ? is this ok, or should i be looking to adjust the antenna or counterpoise to a better match on 20 or maybe I got it wrong again ?

2 Likes

Hi Tim,
You would have to use a different UNUN (49:1) but a 40m EFHW will work on 20 & 10m as well.
Checkout Rob’s antenna calculator site here:

73 Ed.

3 Likes

Your dimensions correspond to the W3EDP antenna… reserche at www. - With these antennas often only use 1:4 baluns .

73 Armin

4 Likes

Tim, I use a similar set up. The W3EDP and 9:1 should give good SWR readings on 20m without a tuner. I use a 9:1 with mine, with the 9:1 connected straight to the radio. Maybe you have a bad connection.

If you want to try something shorter and therefore easier to manage, then try 41’ of radiator and 17’ of counterpoise.

As for the 49:1 and EFHW, well that’s a different kettle of fish. Resonant on 40, 20, 15 and 10m with no tuner, but will work well on 17m and 60m with a tuner.

7 Likes

Thanks @MM0EFI. very useful to know you don’t use a tuner with 20m and this tallies with comments from others (and maybe if i just checked the SWR first :wink: ) Did you cut your wire to have good readings on 20 ? or just cut 84ft. I guess the choke/unun box adds electrically to the length.

interested to know if you need to choke yours at all for such a setup ? im guessing not because the 9:1 is straight to the radio. i’ve had bad experiences with the xiegu and common mode current…

49:1 / EFHW is in the works but not there yet.

1 Like

As Armin says, the dimensions are those of a W3EDP antenna. Years ago G3BDQ took a close look at the W3EDP and found that the 17 ft (5.18m) counterpoise was good for 3,5, 7.0 and 21 MHz but for 14 and 18 MHz it needed to be shortened to 6.5 ft (1.98m), lengthened to 23 ft (7.0m) for 10 MHz and no counterpoise was needed for 24 and 28 MHz. However for these dimensions he used an ATU instead of the UNUN. The tuner sounds like an unnecessary complication but using a small T or Z match and a few small switches to change the counterpoise length would give you a genuine 8-band antenna that has passed the test of time. In fact it could be a nine band antenna because a 126 ft counterpoise would make it work reasonably well on 1.8 MHz. I should add that the W3EDP has been my favourite /P antenna for at least the last ten years - it might be an ancient and venerable design but it works.

4 Likes

One thing to note is that the feedline is part of the antenna Z. i.e. if you have 5m of feeder after the tuner, then the effective length of the antenna wires changes, so you might need to adjust them. If you look at WDEP, the wire lengths are direct to the transmitter tank

3 Likes

Thanks @G8ADD thats useful info and sort of what i was expecting to have to change. That could be quite an easy test to adjust counterpoise lengths and see what dfference it makes.

I should have made clear i’m using a 9:1 unun with the internal tuner / ATU of Xiegu x6100. Apparently its a good ATU and will tune most things…

Yes, thats a good point. i’m now feeding directly to the radio and therefore ATU via the 9:1

The counterpoise is part of the antenna and plays an essential role in SWR. It’s just not just a piece of wire that we can carelessly throw on the ground.

This applies all the more the lower the resistance of the antenna at the feed point, i.e. at the Unun. And that again depends on the ratio of the wavelength to the total length of the counterpoise and the part of the antenna hanging in the air.

With a 1:49 counterpoise is not necessary because in the case of resonance at the feed point the impedance is high and the capacitive resistance of the outer conductor of the feed cable is significantly smaller. Then loss in cointerpoise is negligible.

In other words. What happens if we take a correctly tuned dipole, cut it shorter or make it longer by any length, and place one part on the ground. We lost the part of power, that is radiated on the ground and we have to match its random impedance from 50 to 3kOhm.

73 Chris

2 Likes

Hi Tim,.
Nothing random about your wire, it’s an off centre fed G5RV.

As Ed, DD5LP says, you would be better off with an end fed half wave on 40 m. No counterpoise on the ground, no tuner required for 40 m, tuner touches up swr nicely on 20, 15, 10 m.

It’s smaller, can be erected as an inverted Vee, L, horizontal, etc.

Look up VK1AD’s design notes.

73
Ron
VK3AFW
Edit. VK1NAM.wordpress.com

2 Likes

The different antennas are explained quite well on this page…

I think this can help you

73 Armin

1 Like

We tend to use certain “magic” transformers 4:1,9:1,16:1. These are based on a single turn primary. Since my 43 cores need a 3 turn primary, I have a lots of other Z ratios available eg 7:1 (3:8),9:1(3:9),11:1(3:10)
So this is another variable you have available to find the easiest match for the radio.(well that’s not making it any easier is it?)

Absolutely! It would interesting to see what NEC says about efficiency with it on ground vs 2m high.
If you want to move the feed and counterpoise (say) 2m high with some open feeder, then that 2m of open feeder will change the antenna wire lengths (probably shorten them by ~2m, though knocking 4m off a 30m long antenna would probably be a good thing)

Likely true.
The advantage you might get (i.e. the dream) from a non-resonant antenna + tuner, would be getting 60,40,30,20,17,12,10 from a simple wire antenna with no links or traps etc.(and reasonable efficiency)
An EFHW can’t do this (while working efficiently). I managed to get all bands in two groups with a single coil at the end changing band group, but you still have to get up, walk to the end and change it.

But I can’t say if this W3EDP design can do it either, but for those of you with autotuner radios, it would be a worthwhile thing to do.

Not really, it’s just an OCF dipole, G5RV’s clever idea was find an exact length of open wire transmission line (for one specific dipole length) to match the wildly different impedances of the different bands down to ~50ohms.
Unfortunately for us SOTA ops, that feeder length is around 12m, so G5RV’s are mostly a non starter

2 Likes

… for hanging in free space …
73 Chris

I hit the tune button on my kx2 with every band change.

Last year I tested my 9:1 and W3EDP combo with an antenna analyser. I found that the 20m band had a perfect match without the tuner. Most other bands were below 3:1 SWR, so well within the capabilities of internal ATU’s.

As others have said, this is an ancient design which would have originally been used with robust valve transmitters. Yes, you can shorten the counterpoise here and there and should remove it entirely for 10m, but with your radio (and mine) it’s simpler to leave it in place and tune out any mis-match. No one wants to be faffing about on a summit! My exception to this is the one time I used it on 10m and simply unplugged the counterpoise.

So, going back to my original point. If you can’t get low SWR on 20m, maybe something else needs looking at.

3 Likes

You don’t say how many turns your unn-unn has. The number of turns of the primary and secondary doesn’t only affect the ratio (turns squared) but also the impedance seen by the Tx and the efficiency.

If you are going to try different ratios then it’s worth making one that has taps and a switch so you can use different ratios on each band if needed.

Thinking about that, I reckon its assend backards! The W3EDP dates back to 1935, the G5RV was invented about 20 years later, so the G5RV is a centre fed W3EDP! :upside_down_face:

2 Likes

Thank you everyone for the advice. Lots to think about and an few things to try. Really appreciated.

That is the next step I think, to have a better look. Before i start tinkering.

Thanks for introducing another antenna to some of us!

Interesting to know how all the bands matched esp the low ones.

I did a quick 4Nec2 model of a straight 5.2m:25.6m OCFD at 5m
The lower bands (80,40,30,20) have a low R, suggesting they would be better matched directly than through a transformer.

#W3EDG_1a: R-in (real)
#Freq [MHz] R-in (real) [ohm]

3.6 5.73014
5.36 379.962
7.1 41.734
10.26 144.497
14.3 87.7733
18.1 678.04
21.3 459.932
24.9 1060.66
28.5 1914.27

Since the feed is at the radio, it wouldn’t be any imposition to have a switch.
So if I was experimenting with this (as opposed to just suggesting a whole heap of work for you to do) I would probably have a straight through/transformer switch and taps on the transformer which I can select with a crocodile clip.
An RF current meter in the antenna wire, gives a true indication of radiated power on each band as you change the transformer ratio, and the matching losses change.

1 Like

My real world swr tests yielded the following results, all with 9:1 :

80m 1.3:1
40m 3.3:1
30m 2.2:1
20m 1.4:1
17m 3.7:1
15m 4.0:1
12m 3.4:1
10m 3.2:1* with counterpoise still attached.

Switching un-uns and switched un-uns would negate the need for an ATU, however that is against my ethos of KISS for summit ops.

2 Likes