Sharing a 2M Antenna for scanning

Everything Ham Radio in here, Frequencies, Equipment, Repairs, Mods, Licensing and More

Sharing a 2M Antenna for scanning

Postby Richie_asg1 » Fri Oct 13, 2017 7:35 pm

Hello Folks,

I've recently installed a Radio Shack Pro-2038 in my Landrover on the roof and am using a small FM whip at the moment, but as I have a 2m transceiver working in there I would like to use that larger antenna for the scanner on 2M.

The Trio I am using is old and very basic, using a diode matrix to set the frequencies, has a 20 position rotary switch and S meter but nothing much else. Therefore I can't scan with it.

My plan is to use an antenna switch to give access to the antenna, and use the scanner with it until I hear something, then switch the transceiver on and transmit to reply.....

I think you may be able to see my anticipated problem.

Is there anything out there that allows a dummy load to be substituted for an antenna automatically to prevent me transmitting with no antenna connected?

I'm also not sure if the scanner would like being so close to a transmitting source if I used 2 antennas - and may overload the front end of it. I don't want to suck it and see so would like some advice on this too.

Thanks.
Richard
M6AGL (Magical)
User avatar
Richie_asg1
 
Posts: 19
Joined: Mon Oct 09, 2017 3:22 pm

Re: Sharing a 2M Antenna for scanning

Postby G4RMT » Fri Oct 13, 2017 8:47 pm

My van has 4 antennas, and while they desense each other, so marine goes deaf If I transmit on 2m, it doesn't cause any grief. I've never seen a 4 way rotary switch so you could swap to a dummy load - maybe make one?
G4RMT
 
Posts: 1105
Joined: Sat Feb 07, 2015 5:45 pm

Re: Sharing a 2M Antenna for scanning

Postby Richie_asg1 » Fri Oct 13, 2017 9:09 pm

Thanks for the reply.
Thought I would check before making something up.

I was thinking a circular pressed metal conduit box might be a good starting point, with a chicken head knob on it, and a double wafer switch inside. I suppose bits of a double knife switch may work too, and be more stripline style, but most switches don't bother.

Good to know receivers may not like getting a strong signal but aren't damaged by it. Or at least not the good ones!

Thank you.
User avatar
Richie_asg1
 
Posts: 19
Joined: Mon Oct 09, 2017 3:22 pm

Re: Sharing a 2M Antenna for scanning

Postby m0lsx » Sat Oct 14, 2017 10:45 am

What about a Duplexer? You just connect the Icom to the 2 meter input & the scanner to say the 70cm or 6 meter input & like that both the scanner & the Icom will always be to be connected to an antenna & if the Icom is keyed then the Scanner is disconnected & isolated from the antenna.
A duplexer ONLY activates with RF, so although it says maybe 2 & 70. If you connect you scanner to the 70 port, on RX the duplexer does nothing, the same on 2 meters. It is not until you TX that the duplexer does anything & then it gives the out going RF an antenna & it isolates the radio on RX from the radio TX'ing.
The only thing to watch for is some Duplexers have band pass filters attached to ports, these are not what you want. You want say 30 to 600 mhz for example. Not above or below a certain frequency.
Buy a database from Kimmy JS19 via http://ukscanningdirectory.co.uk/
Or do Google search of this forum via https://www.google.com/cse/home?cx=partner-pub-6291336405621919:2662881632
73 De Alan (M0LSX.)
http://www.qrz.com/db/M0LSX"
User avatar
m0lsx
 
Posts: 5368
Joined: Wed Nov 21, 2012 9:14 pm
Location: Norwich. TG21.

Re: Sharing a 2M Antenna for scanning

Postby Richie_asg1 » Sat Oct 14, 2017 3:38 pm

I hadn't thought about doing it that way.

Most I see use band pass filters so have not seen an active one yet.

I suppose it would be possible to find the Mic PTT switch and use that to switch a transmit/receive relay which would just disconnect the scanner when the 2m rig is transmitting. It wouldn't have to disconnect the transmitter at all so that would be safe on that side of things, but it may start transmitting during those few miliseconds the relay takes to energise directly into the scanner input. Would be safer to use another set of contacts on that relay as the transmit switch, and convert the mike PTT switch to just activate the relay. So when relay is energised it will disconnect the scanner first before it gets a chance to transmit.
Problems then with RF and logic sharing the same relay maybe?
User avatar
Richie_asg1
 
Posts: 19
Joined: Mon Oct 09, 2017 3:22 pm

Re: Sharing a 2M Antenna for scanning

Postby m0lsx » Sat Oct 14, 2017 7:55 pm

Try & keep what ever you do simple. The simpler it is the safer it is, as a whole, as there is less to go wrong.
Buy a database from Kimmy JS19 via http://ukscanningdirectory.co.uk/
Or do Google search of this forum via https://www.google.com/cse/home?cx=partner-pub-6291336405621919:2662881632
73 De Alan (M0LSX.)
http://www.qrz.com/db/M0LSX"
User avatar
m0lsx
 
Posts: 5368
Joined: Wed Nov 21, 2012 9:14 pm
Location: Norwich. TG21.

Re: Sharing a 2M Antenna for scanning

Postby G4RMT » Sat Oct 14, 2017 8:51 pm

I've never had an active duplexer? All mine have been passive devices and in the case of a VHF/UHF one, the VHF port has all the frequencies above about 180MHz chopped right off - at least 90dB of attenuation, and the UHF port gets everything below around 410MHz removed. They have nothing inside that changes with RF input, or at least, none I know of. All of them I have had are just twin filters, and work both ways around - so a dual band transmitter can have two antennas connected or one antenna can go to two separate radios able to work independently. The ones I've had that are quite small, with two cables and one socket are actually a bit lossy - you lose a couple of dB going through them.
G4RMT
 
Posts: 1105
Joined: Sat Feb 07, 2015 5:45 pm

Re: Sharing a 2M Antenna for scanning

Postby m0lsx » Sat Oct 14, 2017 10:39 pm

G4RMT wrote: All mine have been passive devices and in the case of a VHF/UHF one, the VHF port has all the frequencies above about 180MHz chopped right off - at least 90dB of attenuation, and the UHF port gets everything below around 410MHz removed.


I have an old Triplexer, at least I assume it's what it is. It RX's lovely across VHF & UHF on either of the VHF inputs & it allows me to input RF on I think it's 10 meters, 6 meters or 2 meters. I seem to remember picking it up at a table top sale many years ago for a couple of pounds.
Buy a database from Kimmy JS19 via http://ukscanningdirectory.co.uk/
Or do Google search of this forum via https://www.google.com/cse/home?cx=partner-pub-6291336405621919:2662881632
73 De Alan (M0LSX.)
http://www.qrz.com/db/M0LSX"
User avatar
m0lsx
 
Posts: 5368
Joined: Wed Nov 21, 2012 9:14 pm
Location: Norwich. TG21.

Re: Sharing a 2M Antenna for scanning

Postby Richie_asg1 » Sat Oct 14, 2017 11:17 pm

I think I may have a simple solution. I agree with the KISS principle but I also tend to overthink things and that usually ends up more complicated than it should.

I bought an antenna switch that is a 3 way from a common input and looking inside that I can see I could just rewire it to provide a straight-through route for the transmitter and just use the switch to connect the scanner to that when needed. Plus if I change things it would just need rewiring again.

Surprised this has not come up before and there is a standard method to do it. Most transceivers manage it.
User avatar
Richie_asg1
 
Posts: 19
Joined: Mon Oct 09, 2017 3:22 pm

Re: Sharing a 2M Antenna for scanning

Postby G4RMT » Sun Oct 15, 2017 7:30 pm

Would it not just be cheaper and less problematic to just add a 2nd antenna and then not worry at all? twenty quid for a VHF whip and base, with a BNC cable.
G4RMT
 
Posts: 1105
Joined: Sat Feb 07, 2015 5:45 pm

Next

Return to Amateur Radio

Who is online

Users browsing this forum: No registered users and 1 guest