Random Wire Antenna for 160 / 75

[WA2SQQ]

Time for my next experiment.
My property lot does not allow for putting up a dipole for 160, so any antenna I try is going to be a compromise. I also don’t have a large area for a radial field.
I’m currently using a ¼ wave slopper fed against my 48 ft tower. I was thinking of trying a random wire antenna fed with a 9:1 unun. Using the property depth / width / a run to the top of the tower and back down, I can easily loose up to 175 ft. I can also have a few shorter counterpoise elements above the ground.

Assuming I mainly want this antenna for 160 / 75 does anyone have any suggestions for the “optimum” lengths? The feed point will be within 20 ft of the shack, so I’ll use a hefty 1:1 choke to feed the 9:1 unun, hopefully to keep the RF outside. Any thoughts and suggestions?

[KB2WIG]

I'm a poor choice for information, as I'm not on 160, even though I've been attempting to do so for 5+ years........

Look here,

http://rudys.typepad.com/files/qex-ground-systems-part-7.pdf

This guy has a lot of information.

My  advise, is to go to Homely Despot, buy 500 feet of thhn, cut 8 radials ~60 feet long and use  'em as  radials.

klc

[W1ITT]

Stu Perry, W1BB, who is considered to be one of the fathers of 160m DXing used to promote a too-long inverted L antenna.  It turns out that if you use up about 170 or 180 feet of wire total, which is more than the usual quarter wave, the resistive portion of the impedance will be around 50 ohms, but there will be an inductive reactance component.  A series capacitor, moderately wide spaced, of around 200 to 300 pf will cancel the reactance, and you'll end up with a 1:1 swr.  By tuning the capacitor, you can move the SWR dip up or down the band.  As someone else pointed out, some radials are necessary.  If there are yard obstacles to radials, some chicken wire, a 100 foot roll of the 3 foot tall stuff, can be rolled out and a wire soldered on to the ground point of the coax feed.  This can be run along a fence line or under a hedge, out of the way. 
I use some sheep fence as part of the ground on my 160m Tee and work most of what I hear.  Even today's cheapo Tractor Supply farm fence will outlast most of us unless we live on a salt marsh.  And if you live on a salt marsh, your troubles are few in the RF ground department.

[WA2SQQ]

Appreciate all tthe feedback and suggestions.

[WU2D]

Well I finally got something up for 160M. I could have done a quarter wave 135 ft inverted L. I have used that for several years with a tuner in the bulkhead. This year I wanted to try something different so I put up a half Delta. Basically it is 1/3 lambda from the bulkhead (ground level at a 35 degree angle up to the top of a tree then down at a pretty steep angle to ground where I have a ground rod and some radials. So we are talking about approximately 170 feet to the top of the tree (at 75 feet) and 85 feet back down.

It tunes with a coil of about 10 turns of 3" Diameter spaced tubing and 450 pF to ground.

We shall see how it does Tuesday evening on the Grey Hair Net!

[RolandSWL]

Hi Bob,

It 'aint a random wire but it will fit on a small lot. See the antenna section under "160 m elevated aluminum tubing magnetic loop"

Roland......

[WA2SQQ]

"to run a kilowatt, the capacitor needs to withstand 30kV or so, and 100 amps"

Yikes! That's lethal and these tend to be very narrow banded, at best a few khz.

[flintstone mop]

Hi Bob,

It 'aint a random wire but it will fit on a small lot. See the antenna section under "160 m elevated aluminum tubing magnetic loop"

Roland......

Saw the mag loop antenna. She'll work. A lot of work obtaining the pipe and there's some expense with a vacuum cap and 'tuning motor'.... I built a mag loop antenna using a different design of wrapping a foil helically around PVC , and a vac variable. For 160m the diameter would have to be much larger than the 72 inch dia I used.
At 160m current and KV's are the big stress on a mag loop

[RolandSWL]

O.K. so no death-ray loop.

Another, less lethal, antenna:

http://www.okdxf.eu/files/Olinger_FCP_article_as_published_in_NCJ.pdf..

R.........

[WA2SQQ]

Looks interesting - good reading for this evening's train ride home.

[WU2D]

Uh Oh, the Half Delta is acting like a badly terminated half wave inverted L. Why? Because the end is not actually grounded. The whole idea of this antenna is to FORCE the end to ground thus making it a high current point where the radiation occurs. And if you are doing things right, because of the angle it actually approaches a 50 Ohm Z on the feed side.

Antennas are controversial and fun because everybody can make bad ones, good ones, modeled ones, lucky ones and endlessly talk about them.

I have attached a simple diagram describing some ways of feeding a Marconi Antenna, the classic low angle radiator for the low bands. Everybody agrees that this antenna is the ticket on 160M.  We know about A - you make the quarter wave a bit longer and feed it with a variable cap to tune out the inductive component and it feeds from 50 Ohm coax! Of course it does not actually radiate because we usually have no decent connection to ground, but that is another subject!

In B we have the classic shunt fed flagpole or tower - sort of a Gamma match affair. In C we have the second or third floor dweller who decides to top feed. Hey! I think we can assume that the top of a vertical is as good a place to feed as the bottom, maybe better. It will be a couple thousand ohms so open wire feed and a balanced tuner works just like it would for say, an extended Zepp.

Finally we have a single wire feed off an L match. If this is made a half wavelength long, you essentially put high voltage at the top of the vertical where you want it. This is just like feeding an end fed half wave. But the half wave section will radiate too in the middle, so the pattern will suffer (This could be useful depending on your situation).

So if we can agree that these 4 feed methods are equivalent...

Now back to the Half Delta which is essentially a half wave antenna in total length but only a portion of it is vertical. If you truly have a good ground on the base of the vertical out at the end, you certainly should not have a high impedance at the feed end! BUT I DO. So I know that I am not grounded properly.

Does it work? Well it works better than a wet noodle, but it sure is not right according to signal reports on the Grey Hair Net. I am over 9 on the Cape which is about 200 miles out and they hear me in Maine and New Jersey. But a low Dipole can do that...

I said I was not going to get into grounding - but as with all Marconi's, this is the real discussion. My present inferior system is a single ground rod and a quarter wave counterpoise off it running back along the wire from the rod on the ground.
 

[WU2D]

Well I lifted the end off ground to an almost horizontal orientation as a half wave long wire inverted L. And of course the feed impedance went way up as the end of an open half wave should. So I had about 500 Ohms based on my LC match with the end grounded (it should be closer to 50 ohms! if I was really grounded) and over 2500 Ohms with it as a long wire. I am going to try it tonight as a long wire on 160M. I have installed a single 135 ft counterpoise at about 20 feet from the bulkhead out. I am not expecting much.

Oh but now it works "OK" on 75 Meters as a full wave where as a grounded half-delta, it was no good on 75, at about 10 dB down from my loop. Now it is neck and neck with the loop on stations off the end but is a bit noisy on 75m compared to the vertical loop.

My next move will be getting the Half Delta set up right this weekend. The RF ammeter is coming out.

[w1zb]

Mike - I heard you on the Grey Hair Net tonight and you were 20/9 into Andover MA. A very good signal given the band conditions.
I am also using a end-fed half wave 160M antenna and L Network tuner here. I played around with different grounding configurations and found that a ground rod and ground connection to my home's baseboard radiator system worked the best for me. I also tried a 1/4 radial but it did not do much for my transmitted signal. Maybe I need to try the various grounding combinations again.

73 Jerry W1ZB

[WB4AIO]

I use a 150' inverted L. If I went the 175' route, I could use the series capacitor matching method and the performance would be little different.

The 150' L uses less than 80 feet of horizontal space (70 feet sort of vertical, 80 feet more or less horizontal). Fifty feet up and 100 feet horizontal would be fine too; I've modeled it. And it works very well for my purposes -- get great reports all up and down the eastern half of North America from here in western PA.

My antenna is not connected to ground at all and has no radials on the ground. It uses just TWO elevated radials, each 50 feet long and in perfect 180 degree opposition to each other. The radials and base feed point of the antenna are eight feet in the air.

All is supported by trees and the wires touch the trees at many points; makes no discernible difference in performance.

Using two current baluns, one at each end, I use open wire feed and match the system with a T-network tuner in the shack. But you could just as easily use coax feed and a tuner at the base if you prefer. It is also a very good antenna for 75, 40, and 20.

73,

Kevin.