Re: [Elecraft] OT: Advice re: blocking RF from rx-only antenna
As one who deals with isolating very sensitive low-noise preamps from QRO levels at VHF I can offer a little more background on receiving on the same antenna as one transmits. At HF probably most coaxial relays exhibit adequate isolation, but most eme'rs are familiar with the situation that it isn't necessarily so at VHF and higher where relay specs are derated. Your goal should be that the receiver never sees RF above 0 dBm (1 mw). So how much isolation does the relay require if you run 500w? 500w = 57 dBm, so you need at least 57 dB isolation. Many relays only offer about 45-dB isolation so check the specs. They may state good for a Kw or more but that is the transmit rating and not isolation. I initially used Tohtsu CX-600NL relays rated at 1200w (but only had 45 dB isolation). That meant my expensive low-noise preamp was seeing +7dBm (5mw) which is marginal. I upgraded to relays with over 100-dB isolation for use with my 1400w 2m-8877 amp. At these levels it is most important that the relay is not switched while 1400w RF is flowing thru it (called hot switching). A sequencer is the absolute solution for that. MY sequencer also detects relay position to enable the K3 via the inhibit line. Last week I had a failure on that ckt and no RF was transmitted (worked as designed). I had forethought and added an inhibit bypass switch which allowed me to verify if it was a relay issue of just control failure ( SWR was fine indicating a failure in the inhibit control ckt). Locating preamps at the tower top inside the near field of QRO antenna arrays has additional problems. I ended up using double-shielded coax from the TR relay to the input of the preamp and shielded 12vdc cable to prevent RF pickup on the power leads. This in addition to RF chokes and bypass caps being used on the preamp 12v connection. At 600w I did not encounter these issues! 73, Ed - Date: Sun, 22 Apr 2012 20:32:17 -0400 From: Don Wilhelm w3...@embarqmail.com Subject: Re: [Elecraft] OT: Advice re: blocking RF from rx-only antenna To: David Herring d...@ah6td.com Cc: elecraft@mailman.qth.net Message-ID: 4f94a311.5050...@embarqmail.com Content-Type: text/plain; charset=ISO-8859-1; format=flowed Dave, That situation is dealt with routinely by VHF folks with mast mounted preamps. Using a coax relay to connect the amplifier to the antenna and a second relay to connect the preamp to a 50 ohm load will usually provide adequate isolation. 73, Don W3FPR 73, Ed - KL7UW, WD2XSH/45 == BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 50-1.1kw?, 144-1.4kw, 432-QRT, 1296-?, 3400-? DUBUS Magazine USA Rep dubus...@gmail.com Kits made by KL7UW http://www.kl7uw.com/kits.htm == __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
[Elecraft] OT: Advice re: blocking RF from rx-only antenna
Hi Everyone, Yes, this is off-topic a bit, but I know there are some of you here who will be able to get me going in the right direction. I have a PixelSatRadio RF Pro 1A receive-only moebius loop antenna. It is being used with a Kenwood TS-440S that does not have the provisions for a receive antenna like the K3 does. In order to make sure one doesn't fry the antenna's pre-amp, I need a fool-proof way to ensure that no one ever accidentally sends RF down the coax to that antenna. I have thought about maybe some kind of a PTT enabled antenna switch. I have thought about some kind black box with a circuit one could build that would act as a gatekeeper to allow RF in from the antenna but block any going out. I have searched the 'net and have not yet found anything along those lines that I could implement. So my question to you is, can anyone here recommend such a switch or such a black box circuit? Or maybe propose something else that I'm not even thinking of? Any ideas, suggestions, recommendations? I need this to be on-the-cheap and I'm not opposed to homebrewing something here, though I only have modest-at-best skills in homebrewing. Many many thanks, Dave, AH6TD I now return this reflector to normal programming, already in progress. ;-) __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] OT: Advice re: blocking RF from rx-only antenna
I used, until recently, a ELAD transmit/receiver switch to protect the front end of my auxiliary receiver. I know that MFJ makes at one model as well (MFJ-1708), perhaps two. The ELAD switch isn't normally available in the US, but is frequently available used and can be ordered from Europe. 73, Mike Alexander - N8MSA amsct...@comcast.net - Original Message - From: David Herring d...@ah6td.com To: elecraft@mailman.qth.net Sent: Sunday, April 22, 2012 4:03:14 PM Subject: [Elecraft] OT: Advice re: blocking RF from rx-only antenna Hi Everyone, Yes, this is off-topic a bit, but I know there are some of you here who will be able to get me going in the right direction. I have a PixelSatRadio RF Pro 1A receive-only moebius loop antenna. It is being used with a Kenwood TS-440S that does not have the provisions for a receive antenna like the K3 does. In order to make sure one doesn't fry the antenna's pre-amp, I need a fool-proof way to ensure that no one ever accidentally sends RF down the coax to that antenna. I have thought about maybe some kind of a PTT enabled antenna switch. I have thought about some kind black box with a circuit one could build that would act as a gatekeeper to allow RF in from the antenna but block any going out. I have searched the 'net and have not yet found anything along those lines that I could implement. So my question to you is, can anyone here recommend such a switch or such a black box circuit? Or maybe propose something else that I'm not even thinking of? Any ideas, suggestions, recommendations? I need this to be on-the-cheap and I'm not opposed to homebrewing something here, though I only have modest-at-best skills in homebrewing. Many many thanks, Dave, AH6TD I now return this reflector to normal programming, already in progress. ;-) __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] OT: Advice re: blocking RF from rx-only antenna
Well, these certainly are compelling solutions, even though they are designed for separate transmitter and receiver, rather than separate transmit receive antennas. I suppose one could just use it backwards so to speak, and it would work. More money than I'd hoped to pay, but not out of the question. Not a bad suggestion really, and I'm not sure how I missed these when I was searching. Thing that I find myself wondering now is what would happen when the receive coax gets shorted to ground, as both these units do when in transmit mode...probably nothing, but that's a question for the antenna manufacturer. Thanks, 73 Aloha, Dave, AH6TD On Apr 22, 2012, at 10:11 AM, amsct...@comcast.net wrote: I used, until recently, a ELAD transmit/receiver switch to protect the front end of my auxiliary receiver. I know that MFJ makes at one model as well (MFJ-1708), perhaps two. The ELAD switch isn't normally available in the US, but is frequently available used and can be ordered from Europe. 73, Mike Alexander - N8MSA amsct...@comcast.net From: David Herring d...@ah6td.com To: elecraft@mailman.qth.net Sent: Sunday, April 22, 2012 4:03:14 PM Subject: [Elecraft] OT: Advice re: blocking RF from rx-only antenna Hi Everyone, Yes, this is off-topic a bit, but I know there are some of you here who will be able to get me going in the right direction. I have a PixelSatRadio RF Pro 1A receive-only moebius loop antenna. It is being used with a Kenwood TS-440S that does not have the provisions for a receive antenna like the K3 does. In order to make sure one doesn't fry the antenna's pre-amp, I need a fool-proof way to ensure that no one ever accidentally sends RF down the coax to that antenna. I have thought about maybe some kind of a PTT enabled antenna switch. I have thought about some kind black box with a circuit one could build that would act as a gatekeeper to allow RF in from the antenna but block any going out. I have searched the 'net and have not yet found anything along those lines that I could implement. So my question to you is, can anyone here recommend such a switch or such a black box circuit? Or maybe propose something else that I'm not even thinking of? Any ideas, suggestions, recommendations? I need this to be on-the-cheap and I'm not opposed to homebrewing something here, though I only have modest-at-best skills in homebrewing. Many many thanks, Dave, AH6TD I now return this reflector to normal programming, already in progress. ;-) __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] OT: Advice re: blocking RF from rx-only antenna
Dave, That situation is dealt with routinely by VHF folks with mast mounted preamps. Using a coax relay to connect the amplifier to the antenna and a second relay to connect the preamp to a 50 ohm load will usually provide adequate isolation. 73, Don W3FPR On 4/22/2012 4:03 PM, David Herring wrote: Hi Everyone, Yes, this is off-topic a bit, but I know there are some of you here who will be able to get me going in the right direction. I have a PixelSatRadio RF Pro 1A receive-only moebius loop antenna. It is being used with a Kenwood TS-440S that does not have the provisions for a receive antenna like the K3 does. In order to make sure one doesn't fry the antenna's pre-amp, I need a fool-proof way to ensure that no one ever accidentally sends RF down the coax to that antenna. I have thought about maybe some kind of a PTT enabled antenna switch. I have thought about some kind black box with a circuit one could build that would act as a gatekeeper to allow RF in from the antenna but block any going out. I have searched the 'net and have not yet found anything along those lines that I could implement. So my question to you is, can anyone here recommend such a switch or such a black box circuit? Or maybe propose something else that I'm not even thinking of? Any ideas, suggestions, recommendations? I need this to be on-the-cheap and I'm not opposed to homebrewing something here, though I only have modest-at-best skills in homebrewing. Many many thanks, Dave, AH6TD I now return this reflector to normal programming, already in progress. ;-) __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
