Multi Mobiles, one Antenna

mavericknet · Post by **mavericknet** » Wed Jul 17, 2002 10:45 am

Ok, here's another newbie question: I have two mobile radios hooked to two antennas, same band/subband. I get feedback from one of the channels on the other radio (different problem don't worry about it here). Here's the question, I want to toss the two unity dipoles and put up one 7dB antenna, and hook another maxtrac to the mix (just for station ID via morse code). Can I just daisy chain all the radios to the same rg8 (I know should use better cable but we're dealing prexisting here) to the same antenna without to much of an adverse reaction (smoke and flames are definatly out of the question).
Thanks in advance.

phrawg · Post by **phrawg** » Wed Jul 17, 2002 12:41 pm

NO NO NO NO NO NO NO NO Then Absolutely NOT !
one radio, one antenna, Phrawg

Nand · Post by **Nand** » Wed Jul 17, 2002 1:49 pm

mavericknet · Post by **mavericknet** » Wed Jul 17, 2002 2:26 pm

Nand, first off, your moniker looks more and more interesting everytime you change it. Second, thanks for the tip, I'll try something else then for the three radios I have to go with, I don't quite care about the morse identifier... it's getting locked in a closet where nobody can play with the mode button. But I really wish I could get two of the radios to link onto the same high gain antenna for broadcast increase... but if it can't get done it can't get done. The key to this idea of daisy chain was cost, I'd decorate the whole roof of our ambulance garage with antennas if they grew on trees.

Thanx

Tom in D.C. · Post by **Tom in D.C.** » Wed Jul 17, 2002 3:29 pm

Mav,

One of the antenna mfrs, it's probably Diamond or Comet, makes a variety of 1X2 and 1X3 duplexers which are reasonably priced and are intended for amateur VHF and UHF transceivers which have separate UHF and VHF antenna connections. A look at this line might do the job for you and save you some money as well.

Tom, W2NJS
...in D.C.

...but who grew up in East Rockaway, LI

radioconsult · Post by **radioconsult** » Wed Jul 17, 2002 3:29 pm

Back in the good old days, i.e., radios that glowed in the dark and had mechanical rx-tx relays you could do multiple radios in the same band with one antenna and still keep the smoke in the radios. You would connect the antenna to the ant terminal on the first radio, then connect a jumper from the rx side of the t-r relay to the second radio ant port, then another jumper to the ant port of the third radio. Then you would run a separate jumper from the rx connectors of all three radios rx input to the last radio and combine all the rx inputs with a small rx multicoupler connected to the rx port of the t-r relay on the last radio. All you had to do was just key the radio you wanted to talk on. Works fine, except for some receiver signal loss through the multicoupler. As an added bonus, if you hade a spare port on the multicoupler you could connect your scanner.
RadioConsult

JohnG · Post by **JohnG** » Wed Jul 17, 2002 7:27 pm

You might also want to check the mobile crossband duplexers made by Maxrad and Larson. These will allow you to use both a VHF and a UHF radio with a single dual-band VHF-UHF antenna. Most of the amateur radio crossband duplexer units tended to have cutoff frequencies that worked for the ham bands, but didn't work well for commercial bands.

I'm using a Larson unit to run VHF APRS and my UHF Astro spectra on a single antenna. As another FYI, a VHF 1/4 wave antenna is also resonant at UHF. You get a slightly funky antenna pattern, but the SWR is fine.

My Larson duplexer was about $70 from Tessco.

mike m · Post by **mike m** » Thu Jul 18, 2002 7:33 am

most of you missed the fact that he said that he is operating both radios in the same frequency band, IE VHF and VHF or UHF and UHF. Have you ever seen a medi-vac helicopter with 5 VHF or 5 UHF radios working all at once with blade antennas 6 inches from one another ? Try a transmit interlock scheme where you have a Transmit/ receive style of relay hooked to each radio and the switched position places a 10 to 20 dB attenuator in line with any receiver when another radio is transmitting, this lowers the sensitivity enough that receiving radios do not unsquelch when another radio is transmitting, the relays are controlled by the PTT line of the transmitters and since you only have 2 radios the logic switching should be easy.

Mike

mike m · Post by **mike m** » Thu Jul 18, 2002 7:37 am

Almost Forgot something, this method of using the transmit interlock was intended that you keep your 2 seperate individual antennas, that is do not combine things into one antenna like every one else mentioned.

Mike

April · Post by **April** » Thu Jul 18, 2002 11:54 am

Also, the newer radios, since the Mitrek days, do NOT have a antenna relay anymore. Most radios now use diode T/R switching which just makes the receiver go nuts with a lot of RF from a nearby transmitter. Even my 900 spectra would go nuts when a quarter mile from old Stanitago Peak (Orange/Riverside cnty, CA). The enormus RF from "S&*% hill" would "light up" any diode switching circuit in your radios.

The usuall is to keep the antennas as far from eachother as posible, vertical seperation is the easiest one over the other.

We did take out the diode switch in a Maxtrac base and put in a mechanical relay which eliminated the diodes getting "lit-up" by the RF.
Even when using the Maxtrac/Radius mobiles in a repeater config., bypassing the RF PA's diode antenna switch and going direct to the receiver input realy helped reduce "junk" getting into the receiver. This is something to try, being a fixed radio location.

Multi Mobiles, one Antenna

Multi Mobiles, one Antenna

Thanks nand

Duplexere and splitters...

something else