Changing feedpoint location on elevated vertical

[KC4YLV]

I am playing around with my Hy-Gain AV-18VS and have a question. By default, this antenna is set up so that the feed point is approximately 16 inches from the bottom of the main vertical element. I would like to remount this vertical in an elevated configuration about 6 feet off the ground, and as such I was wondering if there was any reason I couldn't take the coil and matching assembly and affix it to the bottom of the driven element. This has the effect of moving the coil 18 inches lower, making the taps easier to reach without dragging a stepladder or stool outside every time I change bands.

Any thoughts? Would there be a specific reason they fed it so far from the bottom, or just construction constraints?

[W5DXP]

This has the effect of moving the coil 18 inches lower, making the taps easier to reach without dragging a stepladder or stool outside every time I change bands.

By whatever percentage of a wavelength is 18 inches, that's how much the impedance will change. Hint: Don't do it for 2m where 18 inches is 1/5 of a wavelength. Seriously, you will have to re-tune your antenna for every band where the frequency shift is unacceptable.

[K7FE]

Cecil has it correct as usual. Changing the feed point of the AV-18VS will affect EVERY band and while it is possible, it is just not worth the bother to readjust the antenna for each band. You probably want to enjoy the antenna, not redesign it.

Consider using several relays or a single stepper relay to change the tap on the coil at the base of the antenna. Use short leads. Isolate the DC control feed for the relay coil with about five or six turns through a ferrite bead close to the relay and then control the band changes from your shack.

Also look into mounting the base of the antenna (and radials) high enough so that people can walk under the radials without bumping them. The radials can inflict an RF burn if touched, so if you have family members that might tough low hanging wires, raise them out of reach.

73,
Terry, K7FE

[N8YX]

Cecil has it correct as usual. Changing the feed point of the AV-18VS will affect EVERY band and while it is possible, it is just not worth the bother to readjust the antenna for each band. You probably want to enjoy the antenna, not redesign it.

Consider using several relays or a single stepper relay to change the tap on the coil at the base of the antenna. Use short leads. Isolate the DC control feed for the relay coil with about five or six turns through a ferrite bead close to the relay and then control the band changes from your shack.

Also look into mounting the base of the antenna (and radials) high enough so that people can walk under the radials without bumping them. The radials can inflict an RF burn if touched, so if you have family members that might tough low hanging wires, raise them out of reach.

73,
Terry, K7FE


Could not one utilize a motorized roller inductor / shorting arrangement (such as used in Collins single-wire tuners, for example) to accomplish this remotely?

[KC4YLV]

Good notes all around. Here are my thoughts. A little background on this antenna - it is just 18 feet of aluminum pipe and a B&W coil, center conductor to a tap on the coil, shield to the earth/radial system. So, there are no traps or other considerations. Currently it is roughly tuned to resonate on 20, I get a decent match bottoming out in the 1.2 -1.3 range about 14.150.

Pretty much what I am thinking right now is that I want to dedicate it for 20 and 30 meter usage, which could be facilitated with a single relay and one small coil, rather than the big and corroded coil on there currently. It tunes up nicely on 20m, with good performance, and the coil allows me to travel down to 30. Do you think that at my power levels (barefoot absolute max, more likely 25-50w max, I will never own an amp) I could get away with a standard sized relay to switch the coil in? I'd hate to have to shelf the project because I can't afford a $60 coax relay for an HF antenna - I already have two satellite antennas begging for coax relays, and quite frankly, they get fed first.

[KA4DPO]

It should work just fine as a base loaded vertical. The tap points for resonance will probably be different but the performance may be much better by getting it up higher and operating as a ground plane.

A standard relay should also work fine for 100 watt power level. They used such relays in tranceivers for years.

I say go for it....

[G0GQK]

I would suggest you just leave it as it is. Otherwise you are not going to have fun !!!

G0GQK

[KC4YLV]

Well, that's the problem - "as is" means ground mounted with 16 wire radials. Quite honestly I cannot afford the copper for a 64+ radial system which is what I want. I would much rather build an elevated feed vertical and use a dozen radials split between two bands for better performance :)

That being said, I'm going to just go for it and sink some fence rail, run radials and move the feedpoint down a foot, see where that gets me.

[K7FE]

Everyone admires an experimenter.

Please let us know your results.

I did like one of the suggestions about the motor driven coil by N8YX.

73,
Terry, K7FE

[KA4DPO]

I think you would be pleased with the result if you mount it as a ground plane and move the loading coil down. That is a time proven configuration and I'll bet you a case of beer it will work better on 20 as an elevated ground plane than a ground mounted vertical.......

John.

[W5DXP]

That being said, I'm going to just go for it and sink some fence rail, run radials and move the feedpoint down a foot, see where that gets me.

I may have been confused about your original posting. Are you going to locate the loading coil and part of the antenna below the elevated radials?

[N8YX]

Everyone admires an experimenter.

Please let us know your results.

I did like one of the suggestions about the motor driven coil by N8YX.

73,
Terry, K7FE

As I type this, there's a recently listed Collins roller inductor on Ebay...

[KC4YLV]

I may have been confused about your original posting. Are you going to locate the loading coil and part of the antenna below the elevated radials?

No no, just not two feet up the antenna element! Everything will be 'textbook' base loaded ground plane - feedpoint and coil at the same point as the radials.