AM/FM antenna options in Fire Truck...

LCFD1 · Post by **LCFD1** » Wed Jul 21, 2004 5:28 pm

OK, this might be a dumb question to some...

I have been asked to install an AM/FM stereo in a Pierce. Because of where the customer "must" have the antenna mounted, I dont have a whole lot of space between the roof and the headliner, like 1.25 inches. All of the commercially available car antennas available to me had fairly deep mounting requirements. So, I thought, why not use an NMO mount with just a standard 1/4 wave vhf whip? Im only recieving, so there would obviously be no swr issues.
I went with the 1/4 wave whip because it was available right then, and I wanted to get this project out the door. I also used it because it looked the same as the other vhf whip on the other side of the cab, not that it mattered too much. I crimped a "motorola" style plug on to the end of the RG-58 to fit the stereo.

The problem is, my reception is not so good. I am in the middle of town, so I imagine it will only get worse as I travel away. Any ideas? Is the stereo wanting a 75 ohm input? Could that be a problem? What do you think is the most simple fix here?

One thing I did notice was the motorola plug I used really had no provisions for hooking up the coax braid. I did anyway. Was this a mistake?

This is the first am/fm radio I have installed. It seems like it should be a major no brainer, but I am having problems for some reason.....

Thanks for any input
-Sonny

Zaputil · Post by **Zaputil** » Wed Jul 21, 2004 6:28 pm

A couple of quick thoughs:

Car stereos do use 50 ohm coax. The shield does need grounded.

Remember that the freq. range you are trying to tune is 88-108 MHz. A quater wave whip antenna for that band is around 30" long.

You might not find anthing locally that will fit the NMO mount- you may need to order something. Most mobile antenna manufacuters could probably find a product of theirs that will work.

Good Luck!
-SZ

Tom in D.C. · Post by **Tom in D.C.** » Wed Jul 21, 2004 6:39 pm

Sonny:

1. A 47 mHz antenna with an NMO mounting would probably work very well, although it won't match the other antenna on the roof.

2. Some car radios have trimmer adjustments on them for better antenna matching. It's been a while since I worked on these, and the trimmer might only affect the AM reception, but you could take a look at the back/top/side of the radio chassis to see if your radio has any adjustment points.

wa2zdy · Post by **wa2zdy** » Thu Jul 22, 2004 12:31 am

You could get an NMO-Q base and a whip that fits. The NMO-Q base threads onto the NMO mount and just accepts a whip of your choosing. I think the whip can only be .100" is the restriction. Since there's no matching network in the base, whatever the whip is cut for is the resonant frequency of the antenna.

A quarter wave at 100Mhz is 29 inches, so as stated above, cut the whip for about that. Then as Tom said, there may be a trimmer cap in the radio for AM and that would be the only adjustment. It should work just fine. AM/FM car antennas have no fancy matching networks. They are poorly (usually) manufactured versions of what has been described.

Good luck.

x1sspic · Post by **x1sspic** » Thu Jul 22, 2004 3:22 am

Pop in a CD player. Burn some CDs with rezponse music.

talviar · Post by **talviar** » Thu Jul 22, 2004 4:49 am

Try this antenna

http://www.antennaspecialists.com/lm_cat/lmrpg12.html

Antenna Specialists ASPR790 Series which is a quartwave trim to freq antenna covering 108-512 and uses a standard 3/4" NMO Mount. FM Broadcast at 88-108 is a little below the bottom end and works fairly well.

My agency has one of these on an HME Fire Truck Chassis as well as on a Mobile Command Post with a fiberglass roof (ground plane kit used in the roof) and works about as good as a standard automotive car radio antenna.

Tony

LCFD1 · Post by **LCFD1** » Thu Jul 22, 2004 4:51 am

Thanks for all the replys. It sounds like the length matters more than I thought..... To verify that ordering a longer whip will solve the problem, I think I will give it a try using a stick of stainless welding rod. If that seems to work, I will probably by the real thing.

-Thanks again
-Sonny

WB2IID · Post by **WB2IID** » Thu Jul 22, 2004 8:49 am

I haven't played with any recent AM/FM car radios, but...

Years ago the car installations used a special type of low-capacitance coaxial cable between the antenna and the radio. The cable was actually a shielded plastic tube with a very thin center conductor floating inside.

I tried using standard 50 ohm cable once and it absolutely killed the AM reception. It may even affect the FM as well, I don't know.

The trimmer referred to above was used to compensate for the cable capacitance on the AM band. The RG-58 coaxial cable has about 30 pF per foot of length and can't be tuned out.

You might try soldering the cable from a standard AM/FM automotive antenna to your NMO mount and use some cable extenders. You should be able to try this outside the vehicle to see that it works before doing the install.

Joe

joe1234 · Post by **joe1234** » Thu Jul 22, 2004 9:04 am

Is It a firber glass roof if so make sure the outer sheild is grounded to something but really the radio only uses the center of the coax to recieve...I just put a cd player in a fiberglass boat for a friend and he didn't want a antenna showing b/c he didn't want anything to knock off so I just took a motorola to female pl259 conector and about 20 foot of coax and coiled it up under the dash a it receives like a champ....... i have heard of pepole takeing the center part of the connector and grounding it the the car and the car is the antenna..but I think you might have so interferenc.......just some ideas.......

Znarx · Post by **Znarx** » Thu Jul 22, 2004 11:41 am

go see some heavy equipment manfacturers and buy a ball-mount flex antenna, they are low profile mounting and low profile visually

...Z

Big Blue TO/\/\ · Post by **Big Blue TO/\/\** » Tue Jul 27, 2004 9:26 pm

Sonny-
convince them to install an XM radio. TINY, magnet-mount (with silicone around edges for security) and/or window-mount. It really IS the way to go!

-Happy XM customer OUT.

KC8UWK · Post by **KC8UWK** » Wed Jul 28, 2004 12:11 am

I was told in the old days a wire coat hanger would work

Heath

hooknladder · Post by **hooknladder** » Wed Jul 28, 2004 6:50 am

XM sucks in my area (NW PA). A friend bought a new Silverado with it as a factory option. Cuts in and out all the time. Same with everyone elses. Got a road going between two hills, no tunes. Another friend bought and installed a Sirius system, and no problems there.

Big Blue TO/\/\ · Post by **Big Blue TO/\/\** » Thu Jul 29, 2004 6:03 am

The one thing XM has that Sirus doesn't is terresterial repeaters, and they are adding more in bigger areas. Sorry he has trouble.

Alan · Post by **Alan** » Thu Jul 29, 2004 9:18 am

Did you use 75 ohm coax between the NMO mount and the radio?

Quarterwave · Post by **Quarterwave** » Thu Jul 29, 2004 12:02 pm

I am gathering that you"had" to use the 3/4" hole that was already there...I don't know of anything other than what was already mentioned.
However, I did want to say that Paccar Parts carries an Am/Fm antenna that is used on Kenworth and PeterBuilt trucks with Fiberglass Roofs. It works very well. It is a 30" whip with a small spring, I think the mount was less than 1" deep. 6 or 8 feet of coax and a MOT plug. I have used them on boats before, they are a nice antenna.

So do they listen to the radio on the way to, or from the fire? Must have a big district to cover if they have all that time to listen to music!

Q

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