J-pole antenna question....

601 · Post by **601** » Thu Oct 21, 2004 7:21 pm

Ok, i've been looking around for some info and for some reason can't seem to find it. I just have one simple question I hope someone can answer. I have a 45 watt VHF Radius mobile hooked up to a j-pole antenna that is directly clamped to a metal mast. I want to put up a scanner and exterior FM stereo antenna on the mast also. Both antennas also clamp directly to the mast. Is there any problem with having all 3 antennas with no isolation from the mast? Will this cause any problems. I kinda don't want to just try it, it's an expensive stereo and scanner..... Anyone know about these?

Any help appreciated!

phrawg · Post by **phrawg** » Thu Oct 21, 2004 7:38 pm

No problem. The area of the J-pole that you use to mount with is
essentially "cold" with respect to rf. Just use reasonable spacing
with the j-pole on top and I would put scanner below it on maybe
a arm off to the side then thf fm below that. Just keep the tip
of the scanner ant a reasonable drop below the clamp point of
the j-pole. Maybe a couple of feet or so. Don't see anything
risky about what you need to do. Phrawg

601 · Post by **601** » Fri Oct 22, 2004 7:46 pm

Thanks phrawg, looks like I have a fun and exiting project for the weekend to keep me busy

Thanks for your help, I appreciate it.

RKG · Post by **RKG** » Sat Oct 23, 2004 8:09 am

I'd give serious thought to turning off the FM and scanner before xmitting on the VHF. Not because they share a common ground side, but because of the proximity.

601 · Post by **601** » Sun Oct 24, 2004 9:06 pm

Well, the mast I have them mounted on is 15 feet tall, with the Jpole being clamed on with about 10 inches of pipe. The scanner and FM antenna would be at around the 5 foot mark, leaving about 9 feet of seperation. Is that not enough? I can add on one more 5 foot section if needed.

RKG · Post by **RKG** » Mon Oct 25, 2004 7:26 am

Hitting a receiver tuned for VHF, be it scanner or two-way, with 45 watts emitted from an antenna 9 feet away, even if they are co-linear (so that the receiver antennas are in the null of the transmitting antenna) seems to be to be putting quite a blast on the receiver's front end. I would be concerned about what this would be doing to the life of the receiver.

grinthock · Post by **grinthock** » Mon Oct 25, 2004 8:48 am

Generally i don't think you want a scanner antenna that close. I have heard people say "As long as it's 1/4 wave away" but that (from what I understand) is for SWR not for what you are talking about.

You are gonna overload the scanner's front end no matter where you put that antenna...

So if you want it right beside, go for it... Althought consider some form of antennuator or something so you can block the RF

Run the VHF at low power when you can.

jbxx · Post by **jbxx** » Mon Oct 25, 2004 9:14 am

I believe that it will be dependent on the selectivity,
sensitivity, and ability to reject (redundant) signals,
of the equipment in question. Often when a Motoraola
is compared to say, an Icom, for example, you are talking
about two extreme ends of the possibilities.
J.B. AD6WX