Need mobile combiner for 2 VHF radios onto 1 antenna...

007 · Post by **007** » Fri Jan 02, 2004 6:49 pm

Here's what I want to do:

I am going to be running dual VHF Orions (136-153 and 150-174) in a dual tray/single head configuration. I can only transmit with one at a time, so I want to run both trays into 1 wideband Maxrad 1/4w on my roof instead of 2 seperate antennas.

It saves the RX front ends from getting smoked, and saves on antennas, to run them into a combiner and out to the 1 antenna. Who makes a mobile combiner that can accomodate this request?

See this pic...what is the combiner that CHP is using? They are running their Master-exec VHF repeater and their scanner onto 1 glass mount VHF Antenna Specialist antenna.

jim · Post by **jim** » Fri Jan 02, 2004 9:50 pm

I've seen this pic a million times and still haven't figured out what the scanner relay switchbox does and why it's needed.

firemanbatt · Post by **firemanbatt** » Fri Jan 02, 2004 10:37 pm

I believe that maxrad makes a combiner, but it is more suited to vhf/uhf than vhf/vhf....

apco25 · Post by **apco25** » Fri Jan 02, 2004 10:41 pm

Jimmy,

A combiner won't do what you want because of the frequency spread you want to cover and physical size.

Just mounted the antennas on the roof and trunk if possible to get some vertical isolation. If one is on the ham split and the other on the PS band you should be fine.

If you need isolation you need a tuned cavity to do it and those are not wide band!

mancow · Post by **mancow** » Fri Jan 02, 2004 11:14 pm

Could it be an RF sensing switch? A box that senses the RF and clicks a relay that switches off the scanner side.

I had a kit board that did this.

mancow

nmfire10 · Post by **nmfire10** » Sat Jan 03, 2004 6:52 am

I think thats what it is. If you can't do that, would there be an accessory logic pin that goes active upon radio PTT?? YOu can use that to drive a coaxial relay that diconnects the scanner.

w7com · Post by **w7com** » Sat Jan 03, 2004 7:45 pm

Since he has two transcievers he'll need two coax relays and steering logic.

Here's a place to get the relays http://www.radiodan.com/ted_stuff/tohtsu.htm.

I'm not sure what the GEs provide for switching but his steering logic may be as simple as a few diodes or as much as a few TTL logic chips.

SlimBob · Post by **SlimBob** » Sun Jan 04, 2004 1:18 am

w7com wrote:Since he has two transcievers he'll need two coax relays and steering logic.

Here's a place to get the relays http://www.radiodan.com/ted_stuff/tohtsu.htm.

I'm not sure what the GEs provide for switching but his steering logic may be as simple as a few diodes or as much as a few TTL logic chips.

I've been thinking about this to keep radios from blowing each other sky high -- one on APRS (144.93) and the other anywhere else in the ham band. the idea I had was to run a pair of dummy loads and relays (and seperate antennas). The idea being that if one radio is on the air, the other radio will be shunted into the dummy load. Now, obviously it isn't a problem to have both sides transmitting at the same time, but you would have to do some hackery to keep from nuking your recievers. Really, for something like this, you'd almost want a circulator on each output so you don't trash transistors either.

007 · Post by **007** » Mon Jan 05, 2004 12:05 am

Lemme toss this into the equation...

The 136-153 radio will be 110w, the 150-174 radio is 50w

Our cruisers have an almost identical setup going:

110w Orion 150-174 for voice, with a 5/8w on the roof
50w MDX on 140 MHz for data, with a 1/4 on the trunk

These radios have a minimal amount of interferance with each other, though I imagine that the 110w Orion is nuking the MDX's front end when it is keyed. Since that radio is data, I can't tell the amount of desense on a signal.

The radios in my car are both for voice...

I am not oppposed to 2 antennas...got it already with reasonable spacing (same as above, as Apco25 could attest to)...but I wanted to run them into 1 antenna to save the RX'rs in the radios from nuclear levels of RF when the other was keyed.

If I can't, no biggie....but since CHP is running 2 VHF radios into the "relay box" shown above, it piqued my interest to see if it was feasible for my install.

apco25 · Post by **apco25** » Mon Jan 05, 2004 12:15 am

I think all that box does is disconnect the antenna from the scanner RX when the RANGR is keyed.

IF you want to isolate them you need a tuned cavity which is large. Getting both radios into the same antenna requires a combiner which won't fit in your trunk either!

heh I said that already.

At best try the relay method but then you loose RX on the one radio

007 · Post by **007** » Mon Jan 05, 2004 12:25 am

apco25 wrote:I think all that box does is disconnect the antenna from the scanner RX when the RANGR is keyed.

*snip*

At best try the relay method but then you loose RX on the one radio

It's between the scanner and VHF-Hi Mastr-Exec repeater, not the lowband Rangr.

If I lose RX while one is transmitting, I could care less...can't really listen to the other radio while I'm talking. Too distracting.

If the alternative is the relay box as shown above, then I'll take it. Who makes 'em?

w7com · Post by **w7com** » Mon Jan 05, 2004 12:57 am

The thing is that if your data radio decides to send out some data, the 110w voice radio won't be able to RX. That will cause interupted messages from the dispatcher.

Dispatcher sends a call to the MDC
Dispatcher announces the call on the voice channel
Just at that time the MDC gets its message and sends back an ACK and removes the antenna from the 110w radio. The unit misses part of the call.

If it were two voice radios it would be a good idea. With the voice and data mix it will cause problems. Stick with two antennas. With your (at least) 10MHz split between the two channels you shouldn't have a problem. If you do, pick up a set of mobile VHF duplexers on ebay and tune them to pass the 140MHz (how did you get an assignment there?) and reject the 150MHz stuff on the data radio. I doubt that you'll find 110w cans for the voice side. I don't see a simple way to use one antenna in your situation.

007 · Post by **007** » Mon Jan 05, 2004 1:09 am

w7com wrote:The thing is that if your data radio decides to send out some data, the 110w voice radio won't be able to RX. That will cause interupted messages from the dispatcher.

Dispatcher sends a call to the MDC
Dispatcher announces the call on the voice channel
Just at that time the MDC gets its message and sends back an ACK and removes the antenna from the 110w radio. The unit misses part of the call.

My application will be 2 voice radios....I fixed my post above to indicate this. Sorry for the confusion!

Stick with two antennas. With your (at least) 10MHz split between the two channels you shouldn't have a problem. If you do, pick up a set of mobile VHF duplexers on ebay and tune them to pass the 140MHz (how did you get an assignment there?) and reject the 150MHz stuff on the data radio. I doubt that you'll find 110w cans for the voice side. I don't see a simple way to use one antenna in your situation.

The 2 radios will be for VHF ham and VHF public safety, respectively. The PS radio need only be 50w, since it'll be used for mutual aid channels mostly (and we don't use 'em up here much). The 10 MHz+ split will still be there, but there isn't much room on an Impala for VHF spacing, you know?

I hope this clears it up....both voice radios, no data.

BTW, the 139/142 MHz data allocation is from the fed's (military to be exact). Don't know how we got 'em, but we did...our VHF trunked ASTRO system is on a 139/142 MHz split also.

w7com · Post by **w7com** » Mon Jan 05, 2004 1:19 am

Ok, in that case the simple thing to do would be to have two antennas and then use the PTT signal of radio A to fire the coaxial relay to disconnect the antenna on radio B, and the other way around. If you try to build something to combine the radios to one antenna you may end up with some funky SWR stuff going on or stray RF that will get into the other radio.

I wouldn't worry about it too much though. Remember that CHP was trying to save the front end of a Bearcat scanner, not a commerical rig.

w7com · Post by **w7com** » Mon Jan 05, 2004 1:22 am

BTW: In Snohomish County we perfer SOG tools. Let the Portland guys keep their Leathermans.

http://www.sogknives.com/

007 · Post by **007** » Mon Jan 05, 2004 2:13 am

w7com wrote:BTW: In Snohomish County we perfer SOG tools. Let the Portland guys keep their Leathermans.

http://www.sogknives.com/

Damn...those are spiffy! Never even heard of 'em before.

Radioactive · Post by **Radioactive** » Tue Jan 06, 2004 1:40 am

One thing I have done in the past is mount one antenna UNDER the car. I used DDRR ring antennas that transmit pattern shows as a Vertical although it is Horizontal. With an 110Watt Syntor XX UHF under the car and another 110W Syntor XX UHF on a hidden antenna (looks like an FM radio antenna in it's standard location) on the fender. Worked great! The DDRR used a NMO mount and MUST be located on a flat (floor) surface. Then reset SWR and then on the air. I've since made a few myself, but here is a link to the company:
http://www.untenna.com/

Here is a link to build a Two Meter one:
http://www.antennex.com/preview/2mddrr.htm

Even enough seperation to run two at the same time. Ground must be metal and a good conductor. Some new cars use plastic with a thin metal film in some places, so scrape and make sure. Hope this gives you some ideas that you could use, they worked well for me.

Need mobile combiner for 2 VHF radios onto 1 antenna...

Need mobile combiner for 2 VHF radios onto 1 antenna...

Using a DDRR ring antenna under car.