comet gp-9nc uhf/vhf - adding radials

[wqzw301]

Hello all,
For a couple of years, I have been using a Laird FG series uhf antenna, for my repeater.
Now I am adding a marine radio into the shack.
I Did my research and comet gp-9nc looked like the best dual band, repeater capable, antenna... It is a commercial frequency version of comet's 2m/70cm gp9 antenna and affordable enough.
I also liked that it had the correct length (19") long radials for it's lowest frequency. Only 3 radials tho...
Upon it being delivered I noticed that the 3 radials are threaded (screwed) into a stainless steel disk at the base.
I was thinking about adding 3 more radials to the antenna, to make a total of six...
It would be easy and inexpensive. 3 stainless 1/8" diameter 19" rods, thread the ends with a 1/8 - 20 die.
Drill 3 holes in-between the 3 existing radials and thread them with a 1/8 - 20 tap.
And have 6 radials instead of 3....
I do have circulator/ isolators on the tx of both radios, band pass cavities on the rx sides and a diplexer, to keep the vhf out of the uhf and visa versa.
Any opinions, comments on adding more radials?
Please and Thank You

 

[cmdrwill]

comments on adding more radials?

Yes good idea. I made my radials larger in diameter. .

 

[prcguy]

I think the results of adding more ground radials will be positive and measurable. Will you see or hear any difference on air? No. BTW, I've had two commercial Comet GP-9s and while they are ok in the 460-470Mhz range for GMRS, etc, the VHF side was sharply tuned to 155MHz. I mean sharp like at 154.5MHz and below the match started to get wild and above 155.5MHz the same, so its not useable on the VHF marine band. I still have one in the garage rafters if you want one cheap.

These antennas can also have internal junctions of chrome or nickle plated parts which can introduce IMD in a full duplex system. I would recommend running a full duplex noise figure test with a load on your duplexer, then a known acceptable repeater antenna, then the Comet. If your receiver sensitivity is affected in any way using the Comet then only use it for simplex type use.

 

[wqzw301]

Yes good idea. I made my radials larger in diameter. .

That is fantastic! Great idea. thank you

 

[wqzw301]

I think the results of adding more ground radials will be positive and measurable. Will you see or hear any difference on air? No. BTW, I've had two commercial Comet GP-9s and while they are ok in the 460-470Mhz range for GMRS, etc, the VHF side was sharply tuned to 155MHz. I mean sharp like at 154.5MHz and below the match started to get wild and above 155.5MHz the same, so its not useable on the VHF marine band. I still have one in the garage rafters if you want one cheap.

These antennas can also have internal junctions of chrome or nickle plated parts which can introduce IMD in a full duplex system. I would recommend running a full duplex noise figure test with a load on your duplexer, then a known acceptable repeater antenna, then the Comet. If your receiver sensitivity is affected in any way using the Comet then only use it for simplex type use.

I did test for nickel... but not chrome. Duplex noise test sounds good thank you.

 

[krokus]

Is the modified disk going to handle the mechanical stress of double the physical load being attached, especially with the body being altered?

 

[cmdrwill]

Do you mean where the radials attach?

 

[wqzw301]

I think the results of adding more ground radials will be positive and measurable. Will you see or hear any difference on air? No. BTW, I've had two commercial Comet GP-9s and while they are ok in the 460-470Mhz range for GMRS, etc, the VHF side was sharply tuned to 155MHz. I mean sharp like at 154.5MHz and below the match started to get wild and above 155.5MHz the same, so its not useable on the VHF marine band. I still have one in the garage rafters if you want one cheap.

These antennas can also have internal junctions of chrome or nickle plated parts which can introduce IMD in a full duplex system. I would recommend running a full duplex noise figure test with a load on your duplexer, then a known acceptable repeater antenna, then the Comet. If your receiver sensitivity is affected in any way using the Comet then only use it for simplex type use.

I did take it apart and it was an intermod disaster...... plus wire gouge brass rod.
I gutted it and I'm turning it into a broad band vhf quarter wave with a .5 inch copper tube element.
thanks
d