Antennae in close proximity

[superdeez]

Hello, all. I have a question regarding my home and mobile setups, which are similar in nature.

In both home and mobile, I have both a 2m and an 11m (CB) radio setup. In the car, the 2m and CB antennas are aboue 2.5 feet from each other and at the house they're about 6' apart. If I have a QSO on 2m, I turn the RF Gain on the CB radio down to zero in an attempt to mitigate high levels of RF coming into the radio, but I recently noticed that the CB's meter was rising to a 1 when I keyed up the 2m radio.

Do I still have to worry about cooking the front end of the CB running my 2m rigs at 10W? The antennas in questiion are correct for their respectrive bands. Additionally when mobile, I have a third radio that I just use as a receiver via another 2m antenna, but I have attempted to shield it from high RF levels by placing an RF limiter designed for SDR radios in the coax.

I'm just curious everyone's thoughts if I could be causing equipment damage or anything better I could do. Will putting the RF Gain to zero actually reduce the RF that is reaching the front end of the CB?

 

[Golay]

I think (at the house anyway) that antennas are far enough apart so that won't be an issue. I'm pondering if your issue isn't the antenna, but two feedlines next to each other, one inducing the other. Of course, the simplest situation is to only have one radio on at a time.

 

[W5lz]

What you say is happening is about as 'normal' as it gets and I just don't think you have anything to worry about.

 

[KE5MC]

...snip..
Will putting the RF Gain to zero actually reduce the RF that is reaching the front end of the CB?

It will not reduce RF reaching the front end.

 

[jwt873]

I have my 2M/70CM vertical mounted about 2 feet above my HF beam.. (It's just above the driven element). I've run up to 600 Watts on 10M HF and up to 100 Watts on 2M. There hasn't been any interaction between them.

 

[Hit_Factor]

I run a kilowatt on HF antenna about 6 feet above my VHF antenna. On my truck my two antennas are 3 feet from each other, running upto 100w.

 

[superdeez]

Thank you for the replies. I take it that it shouldn't be an issue if the power is kept low?

 

[W5lz]

I'm not sure low power is the best preventive for one interfering with another until the distance between them get's to be very large. The 'other' antenna is going to have some power put into it. Until it makes a noticeable difference, I don't think you really have anything to worry about.

 

[mmckenna]

I've run a 50 watt VHF radio in my old truck with a 1/4 wave antenna in the center of the roof. 2 feet behind it was a CB antenna. Ran that way for many years, no issue.
I doubt you'll have any problem.

As said, turning down the RF gain won't protect the radio if it was going to be an issue.

But, there are variables. Power, antenna gain, coupling between antennas, feed line losses, durability of the receivers, etc. There really isn't a "right" answer without doing some actual hands on testing.

 

[ab3a]

The front end of some transceivers often have switching diodes, where one diode goes in to conduction for transmitting while the other goes in to blocking to keep the signal from damaging the receiver. These switching diodes may be getting forced in to conduction by a powerful nearby signal. You won't notice much, but you may be reradiating harmonics because of it. So for example, if the two meter radio goes key down and you see some meter movement on the CB, that CB radio may have the switching diode getting forced in and out of conduction by the nearby signal. This could cause you to radiate harmonics on higher bands such as 292 MHz or 438 MHz.

The only way you'd know this was happening is to look for the extraneous signals. If you know someone with a half decent SDR such as an Airspy or a spectrum analyzer you can look for the two meter signal from a distance to see if it meets specification (I think spurious signals are required to be at least 40 dB down from the main signal). You can then disconnect the CB radio from it's antenna and try it again. If you see those spurs disappear you'll know that this is likely a problem.

Most of the antenna systems cited above have horizontal polarization on one antenna and vertical polarization on the other. If they're both horizontal or both vertical, you are likely to get more coupling between the antennas.

This is something that may be so intermittent that nobody notices what's happening. However, if it is happening, it is incumbent upon you to remedy the situation with filters, grounding switches or some solution.

73,

 

[superdeez]

In an effort to protect the CB, I've bought a low pass filter. Since I put this 23 channel unit in I've been getting some minor interference on 2m, so perhaps this will remedy that also. Still waiting for the filter in the mail.

 

[Firekite]

Still waiting for the filter in the mail.

Did anything change with the filter in line?