OCFD Too Close to the 2M Vertical?

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ckmcdonald

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I have a Comet GP-6 2m/70cm vertical up currently. I'm getting ready to put up a 80m OCFD. As most are aware, it can be problematic finding good places to put antennas. The very best place to put my OCFD will leave it hanging in the air about 2' from the center and to the side of the GP-6 (perpendicular).

I've convinced myself that wind will not cause the two antenna's to come into physical contact. However, I'm concerned about RF cross coupling and the potential of damage to the radio connected to the antenna being coupled to.

Is this of concern?

Any other concerns I'm overlooking having antenna's this close together?

The GP-6 is 6.5db gain on 2m and 9db gain on 70cm and I feed it a max of 50W.
The OCFD will be feed 100W.

Thanks
 

jonwienke

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Bad idea. In addition to RF cross-feeding, having antennas within 1/2 wavelength of each other will significantly affect their SWR and radiation patterns.

You'll be risking damage to both radios when you key up, one from high SWR, and the other from excessive signal hitting the receiver. The radios being in different bands will help somewhat, but when TX power is over 50W, antenna placement needs to be carefully considered to prevent radio damage. There's any number of examples on the forums of receiver inputs being blown because the antenna was too close to a transmitter.
 

jwt873

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I don't think it will be a problem considering one is HF and the other VHF UHF. I've never experienced any issues at my place.

I have an 8 foot long vertical antenna for 2M/70CM. It's mounted 2 feet above my HF yagi. (see image below). I run up to 600 Watts on HF and 100 Watts on 2 meters. I've had no problems at all with the proximity. A lot of hams have setups similar to this.

Plus, I have HF in the car. My HF antenna is on the bumper, 4 feet from the VHF antenna which is mounted in the center of the roof. Both antennas are connected to the same radio (an IC-7000) which puts out 100 Watts on HF and 50 Watts on VHF. The radio has been in the car for 5 years and again, no problems.
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prcguy

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I have basically the same setup with a MyAntennas 80m ocfd just a few feet away from a Comet GP-9. The dipole is horizontal and will have little if any effect on the GP-9 and the HF antenna will not know the GP-9 even exists.

I've run full legal limit on the HF antenna and 100w on 2m into the GP-9 and there is no indication of any cross coupling on the radios.
prcguy
 

ckmcdonald

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It would be easy of course to measure the SWR affect.
How would be a good way to measure the RF feedback down the other line (radio disconnected of course)?
 

popnokick

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ckmcdonald- Your setup is very similar to mine. I have a Diamond V2000A for 6M / 2M / 70cm mounted on a shed that is underneath my 160 Meter OCFD. The OCFD is about 20 feet above the Diamond, so a little farther apart than your setup. I have an Icom 7100 that feeds both antennas, one on the 50 mHz and up antenna port, and the other on the HF antenna port. I have had no interactions... however, I do not use separate radios on them. But I would not hesitate to try.
 

prcguy

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You could use a low power wattmeter or RF power meter or RF millivolt meter and load connect to one antenna while you transmit on the other antenna. You will certainly see some power picked up but since HF and 2m are so far apart in frequency that the low pass filter in an HF radio or band pass filter in a 2m radio will reduce the levels to inconsequential levels.

The worst cases of coupling I've seen is a UHF commercial repeater antenna mounted on a tower on Cheyenne Mt in Colorado with an FM broadcast antenna fed with 20kW that showed 4W FM broadcast energy going back into the UHF repeaters duplexer. The other was a UHF amateur repeater antenna on a high power UHF TV tower on Sunset Ridge, So Cal where there was a similar amount of power measured at the duplexer.

In both cases the duplexer seemed to be enough filtering to allow the repeaters to operate with no interference or other problems.
prcguy


It would be easy of course to measure the SWR affect.
How would be a good way to measure the RF feedback down the other line (radio disconnected of course)?
 
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