Crown Vic RF problem

This forum is dedicated to helping people with questions about installing radio equipment in vehicles. This can include antenna installs, electrical wiring questions/problems, and mounting systems. Pictures of installs are welcome.

Note: Discussions regarding lighting, sirens, and other equipment now has its own forum in the 'off-topic' section below.

Moderator: Queue Moderator

Post Reply
WV8VFD
was KD8CPP
Posts: 242
Joined: Tue Apr 17, 2007 8:50 am
What radios do you own?: WAY TO MANY

Crown Vic RF problem

Post by WV8VFD »

I have a 110w Astro Spectra W3 head in my car. The drawr is in the trunk, towards the left on a little wood deck I made of MDF. The antenna is a quarter wave on the right side of the trunk on an L bracket. About 1.8-2.0 to 1 is the lowest I can get the SWR. Im assuming because its a quarter wave, with hardly any ground plane.

My problem is, I have a Signal LCS850 siren/lighting controller. If I key up on high power I get a beep..beep beep..beep...almost like computer speakers when you have a GSM phone near them and they're getting a text message. If I go to low power, you hear a click and a hum, until I un-key, and then you hear another click, and it stops.

Both units are ran to the same ground, a ground bolt I put in. Clean of paint. I have not, however, tried grounding the chassis of the siren. I plan on sanding the coating off of one of the mounting feet, and running a ground wire to a ground. The siren is mounted to the backing of the seat, since I didnt have enough room on the MDF to do a mount for the siren.

Any suggestions? I'm gonna try another guy's 5/8 wave tonight when he gets to the fire house. We have it down to about 1.2:1 SWR, and thats mounted on an L bracket also. Im thinking it might be the SWR getting back in the trunk from the quarter wave. Tried a brand new NMO to make sure the cable wasn't goofed, and both have a real Mini UHF, rather than an adapter.

Any help would be appriciated.

Thanks
Tyler Lewis
User avatar
Andy Corbin
Posts: 158
Joined: Tue Apr 19, 2005 3:46 pm

Re: Crown Vic RF problem

Post by Andy Corbin »

WV8VFD wrote:I have a 110w Astro Spectra W3 head in my car. The drawr is in the trunk, towards the left on a little wood deck I made of MDF. The antenna is a quarter wave on the right side of the trunk on an L bracket. About 1.8-2.0 to 1 is the lowest I can get the SWR. Im assuming because its a quarter wave, with hardly any ground plane.

My problem is, I have a Signal LCS850 siren/lighting controller. If I key up on high power I get a beep..beep beep..beep...almost like computer speakers when you have a GSM phone near them and they're getting a text message. If I go to low power, you hear a click and a hum, until I un-key, and then you hear another click, and it stops.

Both units are ran to the same ground, a ground bolt I put in. Clean of paint. I have not, however, tried grounding the chassis of the siren. I plan on sanding the coating off of one of the mounting feet, and running a ground wire to a ground. The siren is mounted to the backing of the seat, since I didnt have enough room on the MDF to do a mount for the siren.

Any suggestions? I'm gonna try another guy's 5/8 wave tonight when he gets to the fire house. We have it down to about 1.2:1 SWR, and thats mounted on an L bracket also. Im thinking it might be the SWR getting back in the trunk from the quarter wave. Tried a brand new NMO to make sure the cable wasn't goofed, and both have a real Mini UHF, rather than an adapter.

Any help would be appriciated.

Thanks
Obviously getting RF feeding back into the siren unit. Some things to try-

(1) If you have a VHF magnetic mount antenna, try that on the roof of the vehicle. If that solves the problem then its obviously the location of the antenna.
(2) Try some snap on ferrites chokes on the power leads to the siren unit http://www.radioshack.com/product/index ... Id=2103222 or http://www.tentec.com/products/Snap-On- ... Choke.html (lots of other places such as DigiKey, Mouser, etc)
(3) You may need to re-route the power cables to the siren unit
(4) You may need to re-route the antenna coax cable
(5) You may need to lower the power in addition to any or all of the above suggestions. Lowering the power to 50 watts will probably have negligible affect on performance.
User avatar
wx4cbh
Posts: 337
Joined: Tue Aug 05, 2003 6:01 pm

Re: Crown Vic RF problem

Post by wx4cbh »

In 40 years of experience on my part in mobile installations I've seen numerous cases where RF can get into the power and control cables on sirens and cameras, etc., when the SWR on a given antenna system is above nominal bounds and any portion of the RF cables and the power/control wiring is routed in parallel, even for a short distance. I like neat orderly cable routing as much as anyone, but the rule of induction is that if RF and power and control cables are routed in proximity to each other, ideally they need to cross instead of being parallel, so I agree wholeheartedly with the thought that you may have to reroute all the cables. Since that isn't practical in most cases, they should at least be separated as much as possible and laid in a twisted pattern so that none of the cables are actually parallel. Our policy is to route power and control wires as far as possible from RF cables. The majority of the time this practice is more than sufficient unless there is a real SWR problem, and resolving that always seems to fix the uglies. I have actually had to route cables in a loose braid to accommodate the non-parallel rule in very limited spaces.

The higher the SWR, the more skewed the radiation pattern and the radiation efficiency will be, and the more likely there are to be radiated and induced anomilies. Curing that SWR problem alone may actually eliminate the problem you are seeing without further action, however, none of the above has actually addressed why the SWR of the quarter wave antenna cannot be gotten down to 1.2:1 or lower on your center frequency. A clue that something is awry with your current setup is that a quarter wave antenna is generally much more forgiving in less than ideal mounting circumstances than a gain antenna is. Substituting a mag mount won't tell you if there is a cable routing problem, but since it's simple, quick, and dirty, you have absolutely nothing to lose by trying it as a beginning step. If you are lucky enough to have an antenna analyzer available to you, that should give you a good idea of what the real problem is with your current antenna and feedline setup.
curmudgeon.....and I like it.
Jim202
Posts: 3609
Joined: Sun Sep 09, 2001 4:00 pm

Re: Crown Vic RF problem

Post by Jim202 »

I have seen where the mini UHF coax connector on the radio antenna port is slightly loose and caused all sorts of problems.

The second problem area is the center pin connection on the radio side of the RF connection. If it goes in real easy, the chances are that the female section fingers are loose and making a poor connection. Again this will cause you untold grief with changing RF connectivity to the antenna. This normally shows up as people will tell you that you have noise or static on your transmissions.

The repair for the loose center connection has two choices. The first is to replace it. The second is a little more daring. I have used a 1/8 or a 3/16 flat blade screwdriver and a small hammer or other blunt object to gently massage the operator end of the screwdriver. The intent here is to put a slight blow on the Teflon insulator about 1/16 inch back from the edge. This generally ends up just about where the small ridge ends. If you do it right, you will have pushed down section on the outside of the female center pin giving it just a slight dip. You might have to use a sharp scribe to open up the outer end of the connector to allow the mini UHF center pin to get a start into the chassis connector. If you didn't over do the de forming, you now have a good mating connection.

Jim
WV8VFD
was KD8CPP
Posts: 242
Joined: Tue Apr 17, 2007 8:50 am
What radios do you own?: WAY TO MANY

Re: Crown Vic RF problem

Post by WV8VFD »

Thanks everyone. I got the SWR down to 1.3:1 then it started going back up. We're going to put chokes on the power, control cable, and speaker wires this week. I put the antenna on a mag mount on the roof, and tried it on the hood. Still making a click on the siren, but no actual noises coming out of it. Funny thing was though, the lower I got the SWR, on the L bracket, the worse it got.

Jim, come to think of it, the connector does seem a little loose, and does go in easy. I'll give that a try, and if that helps, order a new chassis connector from MOL.

No RF or power wires run in parallel. Hopefully I have time to work on this tomorrow. I'll let everyone know how it goes.

Thanks again!
Tyler Lewis
motorola_otaku
Posts: 1854
Joined: Tue Jan 13, 2004 7:03 am

Re: Crown Vic RF problem

Post by motorola_otaku »

I think you mentioned this on Hamsexy, but you have a granny Crown Vic and not a police interceptor, right? That means the trunk lid isn't bonded with ground braid. Run two pieces from the hinge bolts on either side to the inside of the trunk (paint scraped/sanded, of course, and you can spray a little primer paint over it when you're done to keep it from rusting) and you should see at least some reduction in SWR, if not a dramatic improvement. A hole popped in the lid itself with ground braid bonding would almost certainly give you 1.5:1 or better but there's no reason why a properly-installed L-bracket shouldn't work.

You also didn't mention what kind of coax you're using. I've gone to using RG400 for all of my mobile installs after scoring a roll at a hamfest.. it's good for 100W+, it's good for 900 MHz and then some, and 200% shield cures a lot of RF gremlins.
WV8VFD
was KD8CPP
Posts: 242
Joined: Tue Apr 17, 2007 8:50 am
What radios do you own?: WAY TO MANY

Re: Crown Vic RF problem

Post by WV8VFD »

Yeah. Its a granny CV. I'll give that a try too. The coax is just the original coax that comes on a larsen NMO mount. I'll see about some RG400
Tyler Lewis
User avatar
BENWSI
New User
Posts: 20
Joined: Tue Oct 16, 2007 12:09 pm
What radios do you own?: CDM 750 in my truck

Re: Crown Vic RF problem

Post by BENWSI »

We had a similar problem with an XTL5000 handheld controlled trunk mount one of our (Federal gov't) customers. He had a handheld controlled Federal Signal siren. We originally had a high power trunk mount Spectra in the vehicle (Vhf) and swapped for the newer XTL5000. When we did, BAM, we had RF getting into the siren. Spectra NEVER did this, no problem.

These were undercover cars and when you keyed the radio, the siren would "chirp" or the lights would flash, etc. Not great on an undercover car. BLINK BLINK, hey guys, we're over here! :) Anyways, we tried EVERYTHING, and I mean EVERYTHING. The only thing that finally worked was moving the siren unit out of the trunk and under the passenger seat.

We were using a Sti-Co disguised antenna. We swapped it for mag mount Vhf qt. wave, no luck. Moved the antenna completely off the vehicle, no luck. Used a dummy load, no luck. HAD to move the siren. . . period.

This kind of radio
Image
In the Tennessee hill country
Image
User avatar
FireCpt809
Posts: 1846
Joined: Sat Jan 19, 2002 4:00 pm
What radios do you own?: Alot..

Re: Crown Vic RF problem

Post by FireCpt809 »

There was a Service bulletin that came out from Sti-co with using XTL's and sti-co's the RF was getting out from the radio and causing havoc on some of our federal customers with the same set up. It involved using 2 Chokes on the Coax. Call Sti-co and the will fax it to you
WV8VFD
was KD8CPP
Posts: 242
Joined: Tue Apr 17, 2007 8:50 am
What radios do you own?: WAY TO MANY

Re: Crown Vic RF problem

Post by WV8VFD »

Bump.

In all honesty, I haven't really messed with it too much. I put a mag mount on somewhere it wasn't doing it too bad at, but I am wanting to get back to fixing this. I looked at the mini uhf on the radio side, and it is loose as all get out. I am ordering a new connector/cable tomorrow. I also am going to get some ground braid, and ground the trunk. I also seem to think when we installed the siren, we coiled up the excess control cable. I am going to cut the excess, and put a new connector on, so it is a straight run. I am also going to double check the SWR on the antenna.

If that doesn't help, I am going to run the radio or siren power directly to the battery rather than the fuse block in the trunk.
Tyler Lewis
Post Reply

Return to “Vehicle Radio Installs”