Unexpected results trying to match to a low 160m inverted-L in the field, without an inbuilt tuner

Equipment Antennas
1

I’m experimenting with a 160m AM portable station. The transceiver is a homebrew project and runs around 20 to 50 watts of AM carrier depending on battery capacity. If I get enthusiastic I might add CW down the track. Why AM Portable? Why not SSB? Mainly because there’s a healthy AM group here in VK3 and so this rig will be used to call into local AM nets, but I will use it for the occasional novelty activation too.

Now to my antenna challenge.

I am using a 160m inverted-L. Because my Tx is homebrew I do not have the luxury of an inbuilt magic auto tuner/mismatch problem solver! On two outings I’ve tried to throw the weight as high as possible for the vertical section but I have not managed to get it higher than about 8 metres. That is low for a 160m inverted -L. The driven element (vertival /horizontal wire) is a 1/4 wave on 1850 or about 42m.

I modeled it in EZNEC and due to the short vertical section the feedpoint impedance was something like 10 to 12 ohms. Really low.

So I made up a simple L-match consisting of the usual 20 turn air cored tapped inductor wound on a PVC pipe length, and a dual gang variable capacitor with another 900pF (s.m.) in parallel, giving a tuning range of about 900 to 1800pF. The L was in series, the C shunt on the transmitter side.

Here’s what I found in the park today. Threw the wire about 8m into a tree, the rest pretty horizontal to a second tree. Ground was a clip onto the top galvanised wire of a mile long fence, far better than my experiments with a counterpoise. Lots of receiver noise, started hearing the guys in the Net, so far so good.

Couldn’t get the homebrew L-match to improve SWR at all. As soon as I added any L whatsoever, matching was worse and receiver noise dropped.

But, connecting just the dual gang variable cap (with parallel C, a total C of 1.8nF) in series (between the coax inner and driven element) seemed to work, got the SWR right down, and good reports from the other stations I worked.

Antenna afficionados, does that sound right? Seems a LOT of series C, but it appears to have worked. I note that many inverted-L users use longer wires and series C to tune out extra reactance to match 50 ohms. But my wire is only a 1/4 wavelength.

Inverted-L antenna theory is not simple and I am loath to spend a lot of energy to dig into it. I think the two factors in my case are, a) that my wire is 1/4 wave long and b) that the vertical portion is short.

Any comments appreciated.

Paul VK3HN.

2

Hi Paul,
Did you try L in series and C across the tx terminal instead of the antenna terminal? Most L tuners allow the C to switch to the tx side for low impedance antennas.

73
Ron
VK3AFW.

1 Like
3

Hello Ron, thanks for your very fast reply, you must have been lurking here, like me!

Maybe my explanation wasn’t clear. The C was on the transmitter side of the L-match arrangement. So from the coax line, center goes to series-L, and also to variable C to ground. I believe the golden rule is … put the C on the high impedance end of the L.

Paul VK3HN.

4

Hi Paul,
OK. Usually you need some Land C to resonate and transform the load. Auto tuners mostly these days are L matches with the ability to switch the C from one side to the other of the L.

Whatever works best is correct.

In your case the antenna system apparently was slightly inductive so a lot of series C killed that.

I’m wondering if you have plans to take the fence with you next time. It might be the real antenna.

73
Ron

1 Like
5

Thanks Ron. You might be right about the fence. I got a good signal into the VK3KHZ SDR, nearly as good as what I get from my base station. The ‘park’ I was in it on top of a local hill. Radio always has and always will work best when you are on the top of a hill.

Tell me, can you think of any summits in VK3 that have a wire fence running through the activation zone?

1 Like
6

I had a quater wave inverted L at home between trees, probably at about 10m it worked but not brilliantly. To get it to load into 50 ohm coax it seemed to need to be significantly shorter than I was expecting, and it didn’t really improve much with additional radials. From memory…(Relying on my memory probably not a good idea - arrived at one activation with a mains charger and no battery…) it was linked with the capacitance created between the ground and the not very high horizontal part of the antenna. I did play with an antenna analyser on it and ended up more confused - it loaded up but probably was not resonant. PS I now have a 260ft doublet in nothing like a straight line between trees and have used ultralight wire to try and avoid too much sagging. DX-WIRE UL, DX-WIRE Ultraleicht, but must admit I can’t think of many summits it would work on. PPS Lots of summits here with wire fences - but probably a bit of a long journey for a test… ! 73 Paul

2 Likes
7

Mt Gisborne (VC-039). And if it’s not possible to use it, there’s enough space up there to string up a dipole for 630m.

2 Likes
8

Thanks Andrew, never been there, but I see it was quite active over last summer.

1 Like
9

Paul G4IPB, thanks for your comments. I have a 160 inverted-L at home and it works well as a transmitting antenna, and it loads up well without any matching devices. I think the difference is that it goes up about 18m vertically and that makes all the difference. I verified that in EZNEC.

I am keen to use a full sized wire antenna to avoid loading losses inherent in short verticals. I will be operating in parks and wide open spaces so size is no issue. As I said I don’t have a tuner in the Tx, and I want to avoid something big and bulky. I will be running up to 50 watts AM so polyvaricon thingies not suitable. Other 160m antenna suggestions are welcome.

Paul VK3HN.

1 Like
10

A trawl through the index will find several ideas, I’m about to go out so I can’t do it for you now, but G4YSS uses very ingenious tunable loading coils inserted into the band changing connectors in a bandhopper-type antenna, I have used the W3EDP antenna and am working on a loading coil for the counterpoise, and (to my surprise) I found that the humble G6RV used as a doublet is quite effective giving me daytime contacts over some hundreds of miles - but less so in summer.

2 Likes
11

Thanks Brian. I have seen the clip-on loading coils that have been used to electrically extend a linked dipole. I am aiming for a) minimal losses, hence avoiding loading coils, and b) vertical polarisation, as that is favored by the local AM Net stations who I am able to work with this kit.

I guess my constraints eliminate some of the obvious alternatives, maybe I’ll stay with the inverted-L, as long as it is, and continue to experiment with matching it.

1 Like
12

Hi Paul.
A lot of care and attention to the radiator. The ground on the other hand is a totally unknown quantity. Because the radiator is already resonant it is the ground that needs impedance matching. This is why results with the fence as a ground have been poor. A resonant quarter wave elevated radial would give more predictable results and better efficiency.

A higher radiator will help too. I use a tennis ball on a string lofted using a racquet. 15m can be achieved easily as long as the string is light - I use builders line or thin fishing line.

In my own case I use a double-sized ZS6BKW for 160m in the field but it does seem to take a long time to deploy. However, results are excellent. Matching on 160m can be achieved with a series variable capacitor. At 56m long + end strings it is smaller than a half wave dipole.

73, Gerard - VK2IO

2 Likes
13

Thanks for the tennis ball idea, I have been using a throwbag attached to line designed for tree surgeons but was never a good bowler at cricket so it limited the height of the antenna support. Might try tennis! Paul

1 Like
14

G’day Gerard

Thanks for your comments. What is the typical feedpoint impedance of the 2 x ZS6BKW on 160? I am hoping to minimise the size of any matching componentry I need to carry, for obvious reasons.

I layed out a 1/4 wave counterpoise on the ground, under the driven element, and swapped between it and the fence, and the fence wire gave better receiver noise and relative RF power, so for whatever reason it seemed to work.

The tennis ball on a string sounds good, a bit more to carry, but if you can hit it 15m high that would make a decent difference for a field inverted L.

Cheers VK3HN.

1 Like
15

Paul,
There was an article about a loaded vertical for 160m in AR a few years ago, ie. in the last decade, I think by Peter 3YE. He found that a loading coil towards the top of a vertical, with a substantial top hat of several wires going out radially and connected above the loading coil, seemed to work well. I think that may have been the basic design of the vertical made by Tony VK3CAT and used successfully on a SOTA activation with a reasonable ground radial set.

The impedance of the double sized zs6bkw on 160 would be predictably reactive and a high total impedance, based on the impedance of the standard size on 80m, giving an swr of about 8 at the bottom end of the feedline, on mine anyway. If 160 is the only purpose of the antenna, you could forget the feedline length selected by the original designer and go with whatever you found worked. The impedance of the doublet on 1.8 mhz would be quite low, perhaps only several ohms but the reactance quite high. Tuning could be expected to be critical. I suspect the main contribution of the fence to the antenna impedance is a whole lot of ohms. That will broadband any antenna!

Let us know how you go. There will be plenty of observers!

73 Andrew VK1DA/VK2UH

1 Like
16

Andrew, you are correct. I based my 160m Vertical from what I saw of Peter’s at the MDRC a few years back. I then adapted it to what I had available. Pretty sure I have some photos etc on the blog site.
Either way, it is based upon a 8 metre HD squid pole with a 20mm conduit top extension of about a metre. Near the top of the pole is a loading coil and at the very top, angled guy wirea acting as a top hat.
Some basic impedance match tapping at the base.
Tuning can be done by adjusting the angle of the guy wires, effectively being a variable capacitor.
3 radials on the ground at possibly 12.5 m, being 1/2 a roll of wire from Jaycar.
Did work to VK7 on CW from Mildura around this time last year with 10 watts plus phone to central coast VK2.
As for a novelty, have thought of Top Band, Top Mountain.challenge for myself. VK3/VE-001 and other 001 summits on 160m. Probably need an overnight stay.
Tony VK3CAT

1 Like
17

Always used an ATU so never needed to measure the impedance. On the regular size ZS6BKW a single capacitor is all that is needed to transform the impedance close to 50 ohms on 80m. It would be the similar for the double-sized one. In any case a simple matcher will weigh less than the antenna itself.

A 1/4 wave counterpoise on the ground is different to an elevated one. It needs to be elevated so that it offers a low impedance at a known frequency. On the ground it will not be as effective. The fence wire is above ground too but has unknown impedance.

There is an excellent writeup on 160m antennas here: http://k9yc.com/160MPacificon.pdf

73, Gerard - VK2IO

2 Likes
18

Unexpected results are usually caused by losses that are not being appropriately modelled.

4 Likes