Posted: 1/11/2015 12:12:39 PM EDT
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QOTD (Question of the Day): What are the dangers (to equipment - if any) of running 2 HF radios in the same shack at the same time on different bands?
From my thread on switches, I landed on a patch panel, which is working well. I happened upon a conversation this morning on 40 meters where someone was cautioning that the second radio needed to be turned completely off and physically disconnected from the antenna. I missed the first part of the conversation, so perhaps there was a key contextual element I didn't hear (maybe they were talking 2 antennas wired to the same antenna at the same time through a common switch or "T"?). Or, FUD? That convo was enough to get my attention and ask those more knowledgeable. What I'd learned previously was as long as the radios were on separate bands, all was good. I'm generally tuned to 40m SSB on one radio, and want to be running JT65 on another band. |
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Good quality bandpass filters on both radios will make that feasible. That, and the discipline/attention to ensure that both aren't enabled on the same band at the same time. Reactive filters using coils & capacitors usually come to mind first, but I've had success using stubs to notch out harmonics on higher bands. I didn't really understand the concept until I got a spectrum analyzer with a tracking generator though -- I needed to see it for myself on a screen to get that "aha" moment. Odds are you're smarter than me, so you may not need that to make a working stub filter. Another thing that can help a lot is cross-polarized antennas. If one antenna is vertical and the other is horizontal, that cuts interference from antenna-radiated energy a lot, but the small amounts of leakage from feedlines and the radio itself could still be an issue for stations that are in close proximity. Another thing to strive for is keeping the antennas 1/4 wavelength apart (at the lowest freq) if possible. I've had success running 5 to 8 stations at Field Day using nothing more than reactive filters even though antennas were polarized the same and close together. We've even run one station on SSB phone with another running digital on the same band without conflict -- not because of the filters, but because the TX/RX freqs were far enough apart and harmonics are a non-issue when both stations are on the same band. |
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Also, for what it's worth, W1AW has several stations and all their antennas are close together. Inside the building, all the feedlines coming out of the transceivers go to a single patch panel and all the antennas come together at an adjacent patch panel. When you want to operate a station, the attendant picks a bandpass filter for the band you want to operate on and plugs it in between the transceiver and the antenna. That has the side effect of ensuring 2 stations don't run on the same band at the same time because there's only 1 bandpass filter available per band. |
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You can't use the same antenna and coax as it may damage one or both of the rigs.
Running two radio into separate antennas on different bands may cause some interference unless you are using two high end radios with tight roofing filters. Even then it's questionable and depends on how far apart the antennas are and how much TX power each station uses. Many contest stations do this during multi-operator contests. They usually use additional band-pass filters on each radio. I've done this in a contest. My FT-857d was receiving heavy interference from another station but my FTDX3000d worked just fine, with zero interference received. The antennas were about 50 ft apart, both vertically polarized. |
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Thanks, guys - no high end radios here, only an FT-857D and a TS-140S. No bandpass filters or roofing filters, but now I want to read up.
Field Day 2014 was my introduction to all of this, and in addition to being the event that caused me to jump into the deep end and upgrade, that's where I picked up 1) that many people were in close proximity operating, and 2) what I saw was only that they confirmed that everyone was on different bands. There may have been additional coordination / filtering that I didn't see with other groups there, but I know the guy I was with didn't have anything but an 857D and a resonant antenna. My antennas are isolated - one per radio, and there are no switches. My patch panel brings everything close together, but there are no interconnections. While I research more, am I understanding correctly that I may interfere with myself because of the proximity, but am not at risk of damaging the receivers? ETA: Love the drawing! |
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Quoted:
you need compete separation of the two feedlines and antennas look into ARRL FIELD DAY this summer....you'll see lots of HF radios working together in one area <a href="http://smg.photobucket.com/user/derek45/media/RADIO/twoant_zps32734e91.jpg.html" target="_blank">http://img.photobucket.com/albums/v488/derek45/RADIO/twoant_zps32734e91.jpg</a> your basic antenna switch may not have enough separation to keep on radio's TX out of the others RX http://static.dxengineering.com/global/images/prod/mediumlarge/mfj-1702_sn_ml.jpg Go back and sharpen your crayons. Also there is no battery inline with the feed line. Fail. 
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Quoted:
Go back and sharpen your crayons. Also there is no battery inline with the feed line. Fail.
Quoted:
Quoted:
you need compete separation of the two feedlines and antennas look into ARRL FIELD DAY this summer....you'll see lots of HF radios working together in one area <a href="http://smg.photobucket.com/user/derek45/media/RADIO/twoant_zps32734e91.jpg.html" target="_blank">http://img.photobucket.com/albums/v488/derek45/RADIO/twoant_zps32734e91.jpg</a> your basic antenna switch may not have enough separation to keep on radio's TX out of the others RX http://static.dxengineering.com/global/images/prod/mediumlarge/mfj-1702_sn_ml.jpg Go back and sharpen your crayons. Also there is no battery inline with the feed line. Fail.
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Quoted:
My antennas are isolated - one per radio, and there are no switches. My patch panel brings everything close together, but there are no interconnections. While I research more, am I understanding correctly that I may interfere with myself because of the proximity, but am not at risk of damaging the receivers? That's right, it would be unlikely for a nearby ham transmitter to damage a receiver on another band, even without a BPF.. But the lightning battery, that'll make it double safe. 
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At the 100 watt s or less level this should be true, but at QRO power levels you might have a problem.
A high power transmitter on one antenna can result in a potentially damaging RF signal being applied to a receiver on a nearby antenna. When the bands are different, external band-pass filters resolve the problem. But if the transmitter is on the same band, a band-pass filter won’t help and receiver damage can occur. |
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Quoted:
At the 100 watt s or less level this should be true, but at QRO power levels you might have a problem. A high power transmitter on one antenna can result in a potentially damaging RF signal being applied to a receiver on a nearby antenna. When the bands are different, external band-pass filters resolve the problem. But if the transmitter is on the same band, a band-pass filter won’t help and receiver damage can occur. What kind of damage could occur? Would it be reduced sensitivity, complete receive failure, magic smoke release? |
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If your antennas are close together, you will run the risk of damaging receiver parts.
I have run 2 radios (one chasing DX, with the other listening to ALE) without bandpass filtering and eventually the RX went deaf in the ALE radio. Antenna and band switching diodes were shorted, and I don't think it was a coincidence. GL de W1EL |
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*hijack*
I have a Ramsey FM Transmitter running in the shack (very low power). My FM Xmit Dipole is mounted to the same mast as my Scantenna (scanner feed) and desenses the scanners on a few different bands. *I'm thinking* a band-pass filter would help with that...the transmitter is set to 88.1FM. Anyone ever seen anything like that? past that, this timely subject may help with my Winter Field Day try :) |
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Protecting from damage: rough rule of thumb is to ensure no levels exceed +10dBm (10mW) at the receiver input. That's a very conservative number. Array solutions makes a limiter for $55 that is cheap insurance if you are working field day, etc.: AS-RXFEP.
Protecting from interference and desensitization have been well covered above. big bandpass filters on transmit and receive are your friends! |
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Quoted:
*hijack* I have a Ramsey FM Transmitter running in the shack (very low power). My FM Xmit Dipole is mounted to the same mast as my Scantenna (scanner feed) and desenses the scanners on a few different bands. *I'm thinking* a band-pass filter would help with that...the transmitter is set to 88.1FM. Anyone ever seen anything like that? past that, this timely subject may help with my Winter Field Day try :) Since the scanner receiver is very wideband the optimum approach is to attack the problem in two ways. I'd suggest a notch filter at 88.1MHz on the scanner and a corresponding bandpass on the FM transmitter to kill off any harmonics from the 88.1MHz. However if you are only interested in scanning a small portion of the spectrum a bandpass on the scanner might work. However however: how are your antennas arranged? If you can stack them vertically you might get away with it. |