Effects of Steel/Aluminum mast behind GH

[iblackford]

My build plans for an upcoming GH build include bolting a piece of plexiglass to a metal pole that would serve as my mast. The plexiglass would hold all the reflectors, narods, and mounts for the feed element.

The mast would be <1/2" behind the reflector plane, but perpendicular to the reflector elements. Would this adversely affect the antenna? I know most folks here use PVC, but I think steel or aluminum would work well also, and I'd like to try it.

[300ohm]

Which specific model are you building ?

The most important thing is to avoid shorting the colinear reflector rods or the mesh reflector air gap. Also its best to keep the gaps as far away from the mast as practically possible. Inserting some thick plastic or rubber at the u-bolt connection can do that.

[iblackford]

I was going to build a reflector rod DBGH with tophat NAROD's. The mast would run the entire length of the antenna, with some overhang at the bottom to mount in the rotor. The rods would affix to the plexiglass backer, and the backer would then bolt to the mast. I don't plan to use a u-bolt to affix the antenna to a mast, just bolts periodically to affix the plexiglass to the mast. The bolts would be placed so as not to be in line with the reflector rods.

[300ohm]

Sounds kind of fragile to me. Also, the plexi can contract/expand and crack.

[iblackford]

Ok, fragility concerns aside, would the metal mast adversely affect the performance of the antenna?

[300ohm]

The mast would be <1/2" behind the reflector plane

Thats close, and you could lose a little. The antenna would still work fine of course, just not up to full potential. If its overkill for your area, then I wouldnt worry about loss.

[stampeder]

300ohm can correct me, but AFAIK the type of metal used in the pole should not be significant since it is not conducting any of the signal, just reflecting it a bit in free space.

[iblackford]

I'd like to get as close as possible to the design performance out of this antenna. Is there a minimum seperation required to avoid interference? Or, is this akin to stacking and should be modelled?

Thanks for taking the time to answer my questions.

[300ohm]

It should be modeled or better yet experimented with in real time (NEC results with closely spaced wires may not be 100% accurate). Long ago, I modeled the GH with a 1 1/4" steel mast behind it 1" away. No ill effects. But less than 1/2" is a different story. Try to get it 3/4" or more away. Multiple layers is one way, and would add strength.

[300ohm]

300ohm can correct me, but AFAIK the type of metal used in the pole should not be significant since it is not conducting any of the signal, just reflecting it a bit in free space.

Yeah, but Im leary of a NAROD or capacitance type effect on the air gap when 1/2" or less.

[loveota]

I do all my GH builds on the 1 1/2" steel mast. I found in real world testing being within 2" of the mast can cause reception issues such as at 1 frequency the mast can act as a director while at another frequency it can act as a reflector . 3" spacing solved all issues so that`s what I go with . Now mind you I only build reflectorless and no Narods so the effects on those I`m clueless.

[iblackford]

Thanks all,

I'm going to space the mast away from the reflector plane, either with spacers or thicker material.

I'll post my results when complete.

[300ohm]

I found in real world testing being within 2" of the mast can cause reception issues such as at 1 frequency the mast can act as a director while at another frequency it can act as a reflector .

That sounds about right based on my practical experience with other antennas too.

On the positive side, most builds here generally have some overkill, so the effects are minimal for practical purposes. ( although some do under-build).