Isolating Gain Master from poles

[RadioDaze2.0]

I am wondering if there is any advantage to isolating (not just isolating but mkaing the last pole not metal !) a GM from the pole on which it is mounted. I wondered if there would be a case for making the top pole a wooden or fibre glass one ?

Given the antenna is a kind of centre fed 5/8 wave dipole I wonder if metallic mounting poles could negatively influence the balanced nature of the antenna. i.e. the lower radiating element.

(After all you would never mount a centre fed vertical 1/2 wave dipole on a the top of a pole without a boom)

I heard someone say that anything near or close to the built in coaxial choke can negatively affect the antenna, well a pole is very close to the choke. I wondered if the poles could influence the reactive near field of the antenna and consequently the far field.

Sirio make no mention of this in their manual for the antenna.

Any thoughts on this matter appreciated.

 

[gamegetter]

greetings my friend:
i have asked that before, but no harm in discussing....

generally most think that there should not be interference from the mounting pole, but then again Sirio did say no metal within what 10 feet?

be interesting to see what you experience, how long of a dielectric type material pole would you use?

 

[gamegetter]

actual recomendation is 5-6 meters away from any metal objects,

sirio also says amongst other advantages, that:
"Thanks to its cable coil at the base(rf-choke), gainmaster is fully decoupled from its supporting structure(poles,towers etc) with the benefit of preserving both the impedance and the radiation pattern"

 

[RadioDaze2.0]

Thanks for reminding me of that information.

I wonder what "de-coupled" really means though.

We can imagine that the choke electrically suggests that it is the end of the antenna. So the pole is not part of the antenna.

At what stage does a pole become a near field metallic object ? (You can certainly have CMC on your mast with end feds) It is absolutely in the reactive near field of the antenna and as we know the pole has great influence on every other antenna type including dipoles.

Not sure about how long the pole should be I am just wondering what the situation is given the supposed highly negative sensitivity of this antenna to metal (or in fact anything) near by.

I suspect if Sirio found it influenced the antenna significantly I hope they would have mentioned it should be mounted exclusively on a pole made from an insulator.

 

[Marconi]

I am wondering if there is any advantage to isolating (not just isolating but mkaing the last pole not metal !) a GM from the pole on which it is mounted. I wondered if there would be a case for making the top pole a wooden or fibre glass one ?

RD, in the real world my GM decouples the mast from the antenna better that any other antenna I have, bar none. The GM's choke at the base really does an outstanding job with the antenna's metal base attached directly to my metal mast.

Using Eznec modeling I can show you (claim) that my GM decouples the antenna from the mast about as good or better than it shows in the attached model. Here I used a 4" space between the mast and the antenna to represent the choke.

I use to question whether coaxial chokes really worked, but when I saw my GM working like it does for me...I was convinced beyond all doubt.

You do not need to isolate this one any more, you can see in the images what decoupling really means.

 

[RadioDaze2.0]

Cool, a little complex for me with no modelling experience but I think it shows no currents on the mast when decoupling is employed.

Very cool model thanks for sharing that Marconi you are very helpful on this site, we must chat one day : )

Well today with no isolation from my mast I managed 2 contacts into Australia 10,000 miles + with the GM at 25 feet high. So it does something right !

Plus a load of USA stations, Turkey, Tunisia, Brazil, Venezuela, Grenada. Certainly doing the job here at the moment.

 

[Marconi]

RD, you see the important parts of the images, and you see the difference in currents on the mast where they don't belong, right? Also notice the difference in the patterns these two make.

My GM at about 60' to the tip and is broke right now. I've not taken it down yet. I think the top wire that connects to the brass collar in the top came loose due to bending in some high winds.

It has been up over three years however. I hope I can repair it.

It still receives a little better than my Sirio New Top One at 25' feet to the tip, but I don't talk on it any more. I plan to take it down soon and report what I found.

I don't want to loose my Gain Master, probably the best all around antennas I own.

 

[RadioDaze2.0]

WOW... the pattern goes real bad with that common mode issue. Now I realize how hugely important that killing CMC on the coaxial line really is.

Great technical post, thanks again.