Distance Between TX and RX Antennas on House?

[jmooney]

Hi All,

I'd like to install a dual band base antenna (likely no more than 50W at max output) for reporting in to my Skywarn Net and ragchewing. I'd also like to put up a discone or similar wideband RX antenna for monitoring Aircraft, Marine, RR, etc.

I'm assuming I can't put them right next to each other because when I transmit it'll overload the receiver connected to the discone. Is this correct? How far apart should they be on the house?

I guess I have the same question for a random wire or dipole for SWL as I'm considering this too.

Thanks for you advice and help,

Jim

 

[mmckenna]

Hi All,

I'd like to install a dual band base antenna (likely no more than 50W at max output) for reporting in to my Skywarn Net and ragchewing. I'd also like to put up a discone or similar wideband RX antenna for monitoring Aircraft, Marine, RR, etc.

I'm assuming I can't put them right next to each other because when I transmit it'll overload the receiver connected to the discone. Is this correct?

Correct.

How far apart should they be on the house?

More is better, in this case. What I've seen people do is mount the transceiver antenna at one end of the roof and the receiver antennas at the other end. More separation reduces the amount of desensitization the receiver will experience. You can also try mounting the TX antenna well above the RX antennas, if that's an option.

I guess I have the same question for a random wire or dipole for SWL as I'm considering this too.

Usually less of an option due to receiver filtering, but if you are going to run coax to the other end of the roof, might as well run the ones for the SWL antenna, also.

The exact specifics would be hard to nail down. Has to do with transmitter power, feed line loss, antenna separation, filtering on the RX radio, coax loss on that radio, etc. etc. etc. You can experiment and see how close you can get them before it's an issue, or you can just design for maximum separation (considering cable costs, losses, etc) and avoid it as much as you can.

 

[ka3jjz]

This can't be stressed enough - make sure that, God forbid, if your antennas should fall, that they don't fall on power or phone lines. That could be a fatal error, and almost certainly start a fire. Too, power and phone lines (or more properly, things like crappy insulators and transformers) can be noise sources.

Whatever you get for your SWL antenna, make sure it's nice and far away from the home where there is a virtual plethoria of possible noise sources. These are your enemy on HF. If you're squeezed for space - so many of us are - think about loops, which are often mounted at ground level and reasonably easy to get away from the home. Do some homework on this subject, and keep in mind that as long as there's no traps or coils involved, a ham antenna can also be a pretty good SWL antenna (an example would be a G5RV, if you have the space), depending on what you are trying to do

Mike

 

[jonwienke]

I'm assuming I can't put them right next to each other because when I transmit it'll overload the receiver connected to the discone. Is this correct?

If you have the antennas close to each other, you can actually fry the first stage RF amplifier transistors in the scanner, especially if transmitting at 50W or more. I have my scanner and TX antennas about 36" apart on my truck, and when I key up with 25W of power, I get 500mW of RF power feeding into the scanner. That hasn't damaged my 436HP, but I suspect that if I put a more powerful radio in and keyed it up, the scanner would fry.

 

[SCPD]

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Hi Jm....
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Separation and isolation of antennas is as old as radio. Remember the classic transmission-
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"Shut up, Shut up, I'm working Cape Race".... ?
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Sent by the Marconi operator on the Titanic to the nearby Californian, who was operating on a nearby frequency... So much for separation- and for history.
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As you know, the general rule is the more antennas are separated, the greater the isolation- but those variables included frequencies, distances, receiver sensitivity... to come up with an all purpose formulae is all but impossible.

Never-the-less, I'll throw out one to give you an idea of the isolation requirements if, say, your transmitter and receiver are operating ON THE SAME frequency. We'll assume a receiver sensitivity 0.1uV to open the squelch-- that translates to a minimum of 1.9X10-16 Watts using a standard low impedance coax. We'll say the transmitter is 10 Watts;
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Iso = Isolation Requirement in dBs
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Iso = 10 log (10 Watts/ 1.9X10-16 Watts) = 168 dBs.


(.......that's a 1.9X10 to the minus 16th..... my laptop won't cooperate this morning....
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You'd require a 168 dB isolation if they were on the same frequency not to open that squelch-- that is about the attenuation seen in Moon Bounce!
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Of course you don't need or want same frequency operations, but you can see the physics behind all this. Increase the frequency separation will drop the dB's necessary to achieve that hypoetical 0.1 uV sensitivity exponentially.
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But, there is a twist here- achieving that isolation is frequency dependent- the longer the wavelength, the greater the distance (just ask that radio operator on the Californian how far he had to be away from the Titanic not to get chew'd out... ) Also the plane of, and the height difference between antennas is a major factor. For V/UHF antennas you can achieve far greater separation by mounting antennas at differing heights for a same distance as moving them horizontally... this translates to a really major difference.
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Look at the texts on repeater antenna designs if you want more ideas- but its a safe bet that mounting your's at different heights, as far apart as possible, and of course, staying away from each other's frequencies- will work out fine.
..... Experiment !--- it what this ham radio stuff is all about....
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..............................CF
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[majoco]

Seeing as most guys can't talk and listen at the same time ( for CF) perhaps you should sit down and design an antenna disconnect/ground scheme controlled by the PTT line. Some transceivers have a "Rx Out" terminal or similar which is driven by the PTT. There have been RF detectors that will activate a coaxial relay to disconnect the Rx antenna but sooner or later you will pull the wall-wart that supplies the power and......

 

[jmooney]

Thanks for all the advice my friends!

Looking over my roof again and what I'm trying to do, I've come up with a plan where my discone and weather station will be mounted near my DirectTV dish on one end of the house. I live in a Cape Cod style home (slanted ceilings upstairs) but have an upstairs bathroom that extends out and up from the existing roof. I will mount my dual band base antenna on the bathroom to get it above any of the receiving and WX apparatus. I will run the SWL/HF antenna (likely a PAR End Fedz Quad) from a corner of the bathroom (lower than the dual band antenna) to a tree or pole mounted on the shed at the back of my property (a 60ftX100ft lot.) This will keep the V/U transmitting above an RX antennas and the WX station. THE SWL/HF antenna will be between the V/U antenna and the RX antenna and WX station. If I do eventually get on HF here, I wouldn't run any more that 50W at most due to the proximity of the houses. I'd likely operate QRP on digital modes 99.9% of the time. This will keep all antennas and equipment at least 15-20 feet from any power lines or telecom lines so any interference would be minimized.

Take care,

Jim

 

[mmckenna]

Sounds like a good plan.

Is there room to route coax through an attic space, or are you going to route it externally?

 

[bill4long]

How far apart should they be on the house?

Horizontal separation is not nearly as important as vertical separation in this case. Try to get the dual band TX antenna at least 4 meters (2 meters x 2) higher than the RX antenna. The more the better.

 

[SCPD]

"...........most guys can't talk and listen at the same time ( for CF) perhaps....." Smiles at Majoco
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Not to take this too far off the topic but I do this- sort of- all the time. Using remote receiving work sites and differing remote transmitters, I can operate full, same frequency HF duplex in SSB. Of course I'll hear myself on sylabllic bursts- but between them, the other station is clear---the geographical separation attenuates the transmitter's signal so effectively there is no overloading of the remote receivers. Exotic, yes, --- I will leave the transmitter on, and talk thru it like a telephone-- if that make any sense.
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(For what its worth- usually these frequencies are in the 9 and 16 Mhz bands at 3-5Kw, directional antennas....
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...................CF

 

[jmooney]

Sounds like a good plan.

Is there room to route coax through an attic space, or are you going to route it externally?

I'll be routing externally. Not really any attic to speak of so it will be easier this way. Possibly I can use the eaves, I'll have to investigate but initial plans are external and maybe an "entry plate" in place of a basement window.

73,

Jim

 

[mmckenna]

OK, don't overlook grounding/lightning protection, but that's an entirely different conversation.

 

[w6drz]

CF

That is exactly what we do for commercial HF sites. Ten miles is a good rule to use, with hills between if possible.

Craig