Batboard

I'm looking at putting a pair of commercial rigs in the ham bands on the car -- one for APRS (vehicular tracking) and one for general two-way. What's the best spacing for the antennas? One will be fixed at 144.39MHz (probably 1/4 wave), the other more toward the other end of the band 147-148MHz. I'm not abject to using a "dog-dish" wrapped 1/4-wave horizontal antenna on the trunk for the 144.39 APRS system and using my Larsen 3.5 db 2M/70cm whip on the roof for the other. I'm concerned about spacing overall, and if I will get anything out of spacing them.

Also, is there a mobile-ready GPS that can be trunk mounted? I'm not too into the consumer units -- I want something that will perform in a 140-degree trunk without melting down.

For those antennas give yourself at least 17" between the two.

As for the GPS what kind of car do you have? I'm hearing people have finally decoded the OnStar tranmissions. I've been experimenting here with an @Road unit that I picked up at a hamfest. Any trunk mount unit will also need an external antenna so you'll need to find a place for that guy too!

Btw, once you're done we'd love to see pics!

KG6EAQ wrote:For those antennas give yourself at least 17" between the two.

Well, with one on the back half of the middle of the roof of a 94 Caprice, I think an antenna placed at the front edge of the trunk should be about 17", if not that, it's at least 12" vertical.

I'm really keen on using a dogdish to get the -- what, 20db different from changing polarities?, otherwise I'll just use another Larsen Kulrod.

KG6EAQ wrote: As for the GPS what kind of car do you have? I'm hearing people have finally decoded the OnStar tranmissions. I've been experimenting here with an @Road unit that I picked up at a hamfest. Any trunk mount unit will also need an external antenna so you'll need to find a place for that guy too!

I'm using a Garmin GPS2+ in the car, but I'm looking for something right now that can be mounted in the trunk with the rest of the comm radios and run APRS at all times without blowing the other radio out of the water all the time (hence why I'm going to use commercial radios in the ham bands. )

I haven't looked into too many other GPS recievers. I need NEMA at 4800 BPS to be useful, and preferably as bulletproof as possible.

(Anyone got a TOT module for a Mitrek? )

Btw, once you're done we'd love to see pics!

Sure, if it doesn't look like hell I plan to keep my new car pretty clean, but the old car will be the "truck". BTW, driving down a private mountain dirt road by spotlight at 35MPH rocks

Stupid question: where can I get a .100" metal rod at and a replacement setscrew for a Maxrad 5/8's-wave VHF [150MHz] antenna?

I would assume Maxrad sells replacement whips, no? If not, I know Larsen dealers do.

What is a dog-dish antenna? I feel like I'm missing something here. As for it being horizontal, that's 24dB cross polarisation loss. Worth it? That's a lot of loss.

Now for the two radios on 2m together. I'd worry if I were you about one transmitter nuking the front end of the other. Depends how much power you're running, but I succeeded at doing just that as a teenager with a buddy of mine. We used to sit next to each other with our 2m portables - I want to say mine was a Wilson, but not sure now - and be stupid, talking back and forth to each other. I ended up fixing mine when the front end got toasted.

The idea of using two high quality radios is a good one, but since both are on the same band, the front ends are going to pass the RF, unlike it one was high band and the other UHF, where the filtering would keep the out of band RF out. Yes, I know they have protection, back to back diodes, etc, but still . . . are you a gambling man?

Operationally, your APRS rig will only have to transmit; receiving is not an issue. Your main problem, operationally, will be the APRS transmitter desensing your communication radio. But those packets are short and I guess it won't usually be a major conflict. Only you can decide that though.

So . . . that's how I see it. I'm sure others have different ideas.

Your Maxrad parts should be listed on this page.
http://www.randl.com/cgi-local/cart/car ... &sb=1&nh=3

Ron

shaking ok....

This is really simple

minimum horizontal spacing is 1/4w for VHF. Since we're talking the 144-148 split here the spacing would be around 19inches NOT 17 as that is for 150-174 range.

-Since the caprice trunk is HUGE you will have no problem getting the spacing you need.

Or even better put the antennas on the ROOF and the trunk as the vertical spacing will also improve isolation even more.

It is possible to desense the front end of another radio on the same band. However using modern mobile radios (NOT A WILSON PORTABLE) you shouldn't see any signifcant degradation between the voice and data radios.

I have a 110w Spectra and an SM50 on 2 quarters waves on top of the Suburban roof. I've noticed no desense issues.

apco25 wrote:shaking ok....

minimum horizontal spacing is 1/4w for VHF. Since we're talking the 144-148 split here the spacing would be around 19inches NOT 17 as that is for 150-174 range.

that's a duuuur. I probably would have done 17" unless corrected.

-Since the caprice trunk is HUGE you will have no problem getting the spacing you need.

I wish it was as big as a Fleetwood. Two boxes of gear in the rear and some suitcases and it's full.

Or even better put the antennas on the ROOF and the trunk as the vertical spacing will also improve isolation even more.

This is what I had planned on doing, but didn't make clear at the beginning.

It is possible to desense the front end of another radio on the same band. However using modern mobile radios (NOT A WILSON PORTABLE) you shouldn't see any signifcant degradation between the voice and data radios.

I have a 110w Spectra and an SM50 on 2 quarters waves on top of the Suburban roof. I've noticed no desense issues.

I'm planning on a Syntor X or Syntor X 9000 and a Mitrek. I'm a little concerned, as I've read that the Mitrek has some pretty hot oscillators in it and may generate some hard to notch spurs. I know on of my Syntor X's has a terrible spur at ~512MHz. I pretty well get the feeling I'm going to need a bandpass filter on the Syntor X. I'll look again with the other radio that I can turn back up to 100W.

try it without the filter first and see what happens. The front end of the Syntor X and 9000 is pretty tight

apco25 wrote:try it without the filter first and see what happens. The front end of the Syntor X and 9000 is pretty tight

No, the X has a ~512MHz spur on transmit, about -52 down from the carrier at 25W.

Sorry misread what you said.

SlimBob wrote:I'm looking at putting a pair of commercial rigs in the ham bands on the car -- one for APRS (vehicular tracking) and one for general two-way. What's the best spacing for the antennas? One will be fixed at 144.39MHz (probably 1/4 wave), the other more toward the other end of the band 147-148MHz. I'm not abject to using a "dog-dish" wrapped 1/4-wave horizontal antenna on the trunk for the 144.39 APRS system and using my Larsen 3.5 db 2M/70cm whip on the roof for the other. I'm concerned about spacing overall, and if I will get anything out of spacing them.

http://www.untenna.com/model.html <- the 115MHz - 170MHz model is the other antenna I was talking about using.