Using 2 VHF comms on same frequency

[Obes]

I was once told that if you have two VHF radios installed, you should not have them both set to the same frequency as it would be 'unhealthy' for the radio unit itself.
While it makes little sense anyway to have both your radios on the same frequency, i am wondering if it does anything negative to the radio unit?
we normally teach the students to have a relevant frequency on the number 2 set, and to be able to listen in to that anyway, so its not a critical question, but more out of interest. Does anyone know?
The only thing i can think of is that if you have them both set to the same frequency, as you transmit on comm 1, the comm 2 will be receiving that transmission, and maybe somehow that grables the transmission? Any ideas?

Thanks!

[horsebox]

Depending on the proximity and arrangement of the antenna system, you will be exposing the 2nd receiver to very high signal strengths when you transmit on the other radio. There is some potential to damge sensitive components in the receiver circuitry.

[Obes]

That makes sense. Thanks.

[Whopity]

The biggest problem when operating two radios on the same frequency, is feedback, signal from one receiver finding its way to the transmitter completing the loop. A receiver is designed to work alongside the transmitter, they are both in the same box separated only by a pair of relay contacts, so there is little danger of damaging the receiver. In the early days of transistorised receivers some problems were encountered, but that was a long time ago and any receiver should be capable of being operated in close proximity to a transmitter without any problems.

[A and C]

Breakthrough is always a problem with two radios whatever the frequency selected as because of the size of light aircraft the antennas are always near each other.

Some radio manufacturers fit a TX interlock to reduce the breakthrough, this system turns up the squelch on the box that is not transmitting by pulling a pin to ground on the non transmitting box by using the mike key line on the transmitting box.

[ve3id]

The susceptibility of a radio to interference from another on a close frequency cannot be avoided when the antennas are close to each other, and it does not make any difference from the RF point of view whether the radios are on the same frequency or an adjacent one. So you will not damage the radio by having them on the same frequency.

As someone else said, the real problem is audio feedback which would destroy the intelligibility, as well as confusing the speaker!

Received signals can be 'de-sensitised' by the presence of a close transmitter on an adjacent frequency, so you cannot reliably listen to another channel when you are transmitting. The degree of desense depends on the frequency separation as well as other factors such as mixing products in the radio.

Commercial land mobile base stations and repeaters that have to transmit and receive at the same time use cavity filters to get rid of desense in multi-frequency installations, which require complex setup by a skilled technician, knowing all the frequencies in use at that station. The lack of space and the fact the the frequencies are constantly changing makes the use of cavities in an aircraft impossible.

[mrmum]

On a similar vein, does anyone know if it's possible/likely for a transponder to affect a DME? From what I understand the SSR frequency band is 1030/1090 MHz, which lies within the range used by DME. From experience, I've noted that the DME "drops out" mostly when the transponder is in use, more so on some particular DME channels, maybe even depending on the SSR code selected, but I'm not sure about that.
If it could be one affecting the other, is it down to the location of the boxes, or the antennae, or the wiring loom, or (lack of) shielding perhaps.

[Whopity]

Yes it is possible. Normally the antennas and equipment are sited so that there are no problems of incompatibility however; as equipment ages and corrosion sets in, a poor earth connection can give rise to problems. A DC earth is quite different to an RF earth at UHF.

[wigglyamp]

Many DMEs and Transponders are interconnected with a mutual suppression bus to prevent interaction. It's also particularly important to have the transponder and TCAS connected together in this way or the TCAS sees it's own aircraft transponder as a threat!