Stacked discone antennas?

[KC4PSR]

I've seen on eBay what looks like 2 AT-197G/R discone antennas stacked vertically and claiming 3dB gain. I'm no expert but I can't understand how you can stack 2 antennas and phase them properly over such a 220-400MHz bandwidth. Is this possible?

 

[dlwtrunked]

I have one of those. I have not tried it though--I had forgotten I bought it from eBay and put it in my shed. I think that equal lengths go to the same place to a feed line (too much work to get it out of the shed in a rainy night). They are then in phase. The problem would be harder if they wee not vertically stacked and I think that is what you are thinking. But there is still the possibility of not being in phase for signals real close not out from the midpoint but that does not matter for them.

 

[KC4PSR]

I thought that when combining 2 antennas the spacing between the antennas as well as the lengths of the attached feed lines (phasing harness) was frequency/wavelength dependent and relatively narrow. I don't understand how this can be done over such a wide frequency range like the whole UHF air band. Seems the combination would be optimum at one frequency but cancel out at a multiple, i.e when spacing goes from say 1/4 to 1/2 wavelength. 2 antennas combined over a 225-400 Mhz range would have that issue I think unless there's more to this antenna than meets the eye. I asked the seller but they weren't much help. Let me know if you get 'er in the air!

 

[prcguy]

I agree and at one particular frequency you could have 3dB of gain but that should diminish when you get far enough away from the sweet spot of the stacking height and feedline length. If it was a successful design I think you would see more of them and from various companies.
prcguy

I thought that when combining 2 antennas the spacing between the antennas as well as the lengths of the attached feed lines (phasing harness) was frequency/wavelength dependent and relatively narrow. I don't understand how this can be done over such a wide frequency range like the whole UHF air band. Seems the combination would be optimum at one frequency but cancel out at a multiple, i.e when spacing goes from say 1/4 to 1/2 wavelength. 2 antennas combined over a 225-400 Mhz range would have that issue I think unless there's more to this antenna than meets the eye. I asked the seller but they weren't much help. Let me know if you get 'er in the air!

 

[dlwtrunked]

I agree and at one particular frequency you could have 3dB of gain but that should diminish when you get far enough away from the sweet spot of the stacking height and feedline length. If it was a successful design I think you would see more of them and from various companies.
prcguy

The antenna he saw is used by a European military entity and is mil-grade. The spacing rule doe not apply in this case as the antenna is designed for near horizon signals that will arrive at the two antennas in phase. After that, if feed lengths are the same and the impedance is taken care of, there will be no problem. the standard reference on this is Kandoian et. al. in Electrical Communication, June, 1948. but 3dB gain is not much for the effort and probably why it is not common.

 

[KC4PSR]

Thanks guys. Makes sense. It does seem ungainly compared to the sleek "stacked dipoles in a radome" approach. Still would like to hear how it performs if you get yours operational dlwtrunked .

 

[dlwtrunked]

Thanks guys. Makes sense. It does seem ungainly compared to the sleek "stacked dipoles in a radome" approach. Still would like to hear how it performs if you get yours operational dlwtrunked .

I am not sure I will ever set it up--sort of a splurge buy that ended up in my shed after I realized how big it was. Like I mentioned, 3 dB is not much and I know I will not use it to replace the discone at the top of my tower so I am not sure I will ever get a meaningful comparison.