Co-phased firestik II antennas and am/fm antenna interference?

[David Wheeler]

Sorry guys, I've looked and cannot find this in the archives..... so I'll ask

Planning on installing co-phased Firestik II 3' antennas on my 01. The shorter antennas are due to garage clearance issues. Anyway, I'm considering a co-phase setup mounted using Firestik's Dodge fender mounts on both sides. These will put the antennas just a little in front of the doors. I am concerned the am/fm antenna will be too close and cause signal interference. Does anyone have this setup and could comment on performance?

TIA,

David

 

[Gus's 03]

any eliment in close proximity can have a effect, however in this aplication i doubt it will be enough for you to notice.



on a side note i highly recomend running a braided ground strap from the antana mount to a hood bolt on each side to ensure good ground plane

 

[Champane Flight]

Don't know....

I would mount them on the bed rails behind the cab before I would use fender mounts. If you can get them above the cab by a foot or more, your cab will make a great groundplane. I put a set on spring mounts once, they worked great for low clearance areas, a firestick can take a lot of torture.

 

[steved]

No, there are no interference issues... my dad is running 3-foot barjans on the fenders of his 99, he has no trouble with the AM/FM radio. He's putting a out just a little more than stock... around 40 watts peak IIRC...

His antennas are close enough together that the AM/FM and the CB touch sometimes.

steved

 

[steved]

And to add, I'm putting out almost 200 watts, I never have any issues with the AM/FM... my antenna is in the center of the roof.

steved

 

[Champane Flight]

Bootys

I ran almost 600 on the high side, only thing I got was a slight hum from the AM radio when I keyed. I think this was from the alternator trying to keep up..... Oo. I have since changed and put on a headache rack, mounted a single 48" firestick on the center of the rack. Of course this is high, but it talks mucho better...

 

[Hammer]

Lets start with this for you... .



When mounting a set of co-phased CB antenna's you need to think of a few things,width of vehicle,places to mount,roof/airfoil size and other antennas which are close by the mounting place. I will try and address each individually for you...



Distance to mount antennas... . (taken from Firestik's website)

"Spacing of the antennas in a dual antenna installation will have an affect on the radiation pattern of the antennas. Technically, spacing between the antennas should be nine feet (2. 7m) to have the most balanced pattern. If that distance cannot be achieved, that alone should not discourage you from using co-phased antennas as you will still receive most of the benefits of a dual antenna set-up. However, spacing less than five feet (1. 5m) may require some special tuning skills in order to achieve satisfactory SWR readings. "

Now you could tune them if your mounting distance is shorter,but,if you never have done it you will find what a PIA it really is to do. Most time when we mount antennas on a pickup truck we recommend to the owner to run a single.



Mounting Place...

The fender mount is a decent mount,provided that you make sure you have it tightened properly and it will not come loose. Wind-speed and shake will loosen it up over time and you will need to make sure it is checked for tightness. I would not run one on the right side of the truck near the AM/FM antenna due to problems with reflection and image rejection issues with your AM/FM. Having another antenna close to your CB antenna makes it difficult to tune. There is more to consider than SWR's and when another antenna is close it send the Ohm's out of whack and will cause unstable SWR"s. Mount it on the left fender is your best choice.



Another new item out is the bed stake pocket mount made by Firestik. I have mounted a few of them and they work very nice. Usually we have no issues with them and can allow you to run some other style of antennas that the fender mount will not support safely. Here is a link to them for you.

Firestik SS194A Stainless Steel Stake-Hole Mount



While your needed length is due to garage limitations have you ever consider using a quick disconnect for your antenna. Many different brands are available to help those who need to remove them before entering a garage.



For best performance...

The antenna should be mounted in the center of the roof,How you choose to do that is your option. Remember to use the proper length coax cable and stainless steel center studs which will aid in your tuning... ..... Andy

 

[FLynes]

Hammer and Champane can probably confirm this, but a guy who does installs for Clay's Little Radio Shop in Texas (exit 370 on I-20) told me that you want at least 9' of coax too.

 

[MBowser]

Can someone PM me, I would like to set up a system in my truck but don't want to go all out, just something to keep up on traffic and what not while traveling... not really going to be used for a lot of chatting... Thanks

 

[PRout]

They will work ok. The biggest loss will be in the windings of the antennas. Having only 3 ft of antenna length means the coils will cancel out a lot of the effective radiated power.

The nice thing is you dont have spark plug noise to listen to (from your vehicle anyways).

 

[Champane Flight]

Rough estimate..

Hammer and Champane can probably confirm this, but a guy who does installs for Clay's Little Radio Shop in Texas (exit 370 on I-20) told me that you want at least 9' of coax too.

We would always measure coax to wave length of antenna. Usually a co-phased antenna will have a pre-measured coax usually R-58 with it. .

 

[steved]

I guess theory wins over actual use again...

steved

 

[Hammer]

MBowser...

Drop me a note with what you want or desire to do and I will try and help you out.

Hammer and Champagne can probably confirm this, but a guy who does installs for Clay's Little Radio Shop in Texas (exit 370 on I-20) told me that you want at least 9' of coax too.

Actually...

18ft is the best for optimal performance. As Champagne Flight has already noted most all co-phased antenna cables made are of RG58U and utilize two 18 ft leads. Clays is one of the best radio tuning shops in the wild wild west. They had done two radios for me and both performed flawlessly until I moved away from Galaxy radios and since I do not get run the West Coast anymore.

They will work ok. The biggest loss will be in the windings of the antennas. Having only 3 ft of antenna length means the coils will cancel out a lot of the effective radiated power.

Actually. .

The only hindrance in performance on a short antenna is its height. The blind spots created on a vehicle due to the antenna height will do more damage to tuning capabilities and performance than "windings". I have been to many key-down competitions and seen short antennas win in field strength competitions as many times as tall antennas. Properly tuned a short antenna will perform in a field strength test as well as a tall antenna,as long as its properly placed on a vehicle to limit blind spots.

I guess theory wins over actual use again...

Not sure what your implying here but if you meaning we are dissing your thoughts its not true. I am merely telling the truth and have the instruments to show you that if a CB antenna is mounted next to ANY other antenna it will cause SWR issues and image rejection problems. RF does funny things when another antenna is close by the transmitting antenna. Have you ever done a field strength test on that radio which has the antenna next to the FM antenna?. If you did then you would have understood what I meant and not took it as someone questioning what you have experienced. Understanding the theory helps your actual use experience... ... Andy

 

[David Wheeler]

Hey Guys,

Thanks very much for all of your input, advice and personal experiences. You have helped me determine what I need to do.

David

 

[Bodybomber]

I was just at a cb shop that told me that mounting the antennas (2 x4' fiberglass whips) on the front bed rail will not work due to lack of ground plane. Mine are mounted about 3" in from the side of the bed. My swr is inconsistantly high. . Bad connection somewhere?? He sugested adding ground staps between the frame and bed and checking all connections to see what swr's do then I can talk to him about trying different antennas/radios. Is anyone running there antennas on the front rail?

 

[Champane Flight]

Not right

I was just at a cb shop that told me that mounting the antennas (2 x4' fiberglass whips) on the front bed rail will not work due to lack of ground plane. Mine are mounted about 3" in from the side of the bed. My swr is inconsistantly high. . Bad connection somewhere?? He sugested adding ground staps between the frame and bed and checking all connections to see what swr's do then I can talk to him about trying different antennas/radios. Is anyone running there antennas on the front rail?

Ground plane would only matter if antennas upper coils were not above cab. This situation would only effect forward transmision, transmision to the rear would probably be amplified due to reflected power. With four footers you should be well above and cab roof would be a great plane. Then of course antennas hitting the back of the cab would be detrimental to radio performance, a tennis ball on each antenna where possible contact would cure this. What type of mounts are you using? I would lean towards drilling directly through side bed rails, you could mount with a whip spring, but better direct with double fender washers (insulator in center). I have seen a lot of guys mounting full 8' steel whips on the rear rails then folding forwards with tennis balls on contact points. This is sharp looking, but I have never had good luck with whips.



I am running a 4' on center of a headache rack, swr's are 1-1. 5, on high side, flat on low. . Check all connections, all through cab/metal points in coax, antenna bases for ground out conditions, run a ohm meter check for ground between cab and bed (I don't think this is the problem). . Good luck. .

 

[Bodybomber]

Im using k4r stud mounts(bolt comes up from the bottom) with Firestick Fire ring cable and cheapo old Francis fiberglass antennas that I dyed black. I was going to go with Wilson Silver load tuneable tip antennas that have the seperate ground on the base I have never had an antenna hit the back of the cab.

 

[ToolManTimTaylor]

I know this is all set said and done but the way I mounted mine is using TWO fender mounts and ground preped them and have sandwiched them between the toolbox and the bed rails just behind the cab. They send out way further than they recieve I can tell you that. I do however have an issue with my subwoofer amp. The coax runs down the driver's side and the CB is mounted in the glovebox with an external in cab speaker just below the cupholders.



My amp and all stereo cables run down the center of the truck and my audio cables run down the passenger side. When the mic gets keyed the sub thumps loud enough that I am willing to bet it is hitting max excursion. When using the CB I have the stereo off so this won't happen. I have my SWR dang close to perfect and really can't figure it out.



Do you guys think that I need to obtain a longer coax and make a figure eight loop and tuck it behind the rear seat in the center?

 

[Bodybomber]

I just put the ground straps on and it didnt make much differance at all. Ch40 went from 1. 8 to 1. 7 Ch1 was 2. 7 to 2. 6. ch 20 is now 2. 2. My previous intermitant swr in the red readings was caused by a dirty switch on the meter. The antennas are mounted on stud mounts through the front bed rail. Will a tunable antenna be able to get my swr down enough to make it worth while?

 

[Champane Flight]

Interesting

The problem with the thump on your subwoofer is interesting, are you running boots (Linear Booster) on your CB? You should really separate your cophased coax, running it down one side may be concentrating reflective signal strength, therefore setting off sub woofer? Run it down both sides of truck, maybe under headliner, do your figure eightloops in odd lengths to take up slack.



BB, Your swr's are on a backwards lean telling me that antennas are probably tuned wrong now, trying to trim them anymore would take them further out. I would check insulators on mounts, and put a new set of antennas on, do not tune them until you get a solid signal (no intermitant jumps). Your SWRs are OK, but not optimal, I have run up to 2. 5 on the high side, but this was running some heavy wattage, with stacked amps...