Antenna Spacing

Discussion in 'Equipment' started by Tom Stefanac, Jan 27, 2007.

  1. Tom Stefanac

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    I know I know.. the more antenna's the better, it is best to have a good 75 watt 2m radio strictly for simplex with a nice 5/8 whip and a dual-bander so you can listen to skywarn and be ready to report without interrupting your simplex stuff. You probably should also have something else dedicated to APRS and a nice big Wilson 1000 or 5000 for the CB. You might even have a few scanners, one for emergency services and another for EMWIN decoding and wx band scanning. Then of course it is nice to have a good Wilson cellular antenna to pull signal like mad (with a 3 watt amp under the seat). Optionally a satellite dish can be useful to collect rain and on occasion provide entertainment and if you are really into it, why not just throw on two good HF antennas, maybe one for 20 meters and the other for a band of your choice.

    Oh I almost forgot, you still need room for that 2.4 gig Wi-Fi antenna and probably should use something external for that FRS/GMRS radio. Oh and can’t forget the weather station transmitters either!

    Do you guys see where I am going with this? :D

    Has anyone ever had an issue where they have killed the RX on any of their equipment? I don't mean de-sensitized it, I mean burnt out a diode or connector pin from pure RF feeding in.

    Last year I killed the RX on a 250D scanner by keying up 50 watts UHF. The antennas were UHF 1/4 wave apart, in fact they were 1/2 a wave VHF apart (~1m). Yet I still cooked the poor thing and reception on all the bands is pretty poor with 700 meg - 1.3 gig suffering the most.

    With roof space on most chase vehicles at a premium it is easy to see why cooking stuff becomes a concern, in particular amateur gear and scanners. I personally have not been able to cook the CB, cellular or wi-fi stuff yet but there is always a first I am sure!

    So, does anyone have bad experiences or similar concerns? I see some guys with antennas like 2 inches apart LOL but it is not RF, what if they touch when one is keyed up? :eek: LOLOLOL
     
  2. Andrew Herron

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    I'm going to forward this post to my father. He's got 5 antennas on his truck without issue. I'll get back to you by Monday with his setup and recommendations.
     
  3. Tim Stoecklein

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    I'd say you have too many antennas, but that's just me ;) Really though, I've not even noticed an issue with the distance between antennas being more than a couple of feet. I have experienced some desens on my 2m a couple of times, but that was running HF at 100watts on 20m. I rarely even hear a click on my scanner (if it's in there) when I key up my dual-bander at 50watts.

    I'd say it depends more on the sensitivity of your equipments' receivers than it does with antenna proximity. Good luck with the porcupine farm!
     
  4. Mike McDonald

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    I used to drive a Suburban that had 8 antennas on the roof. Now I have a quad cab pickup with 5. Occasional intermod is a problem, but that's the only problem I've had. I know a couple of people that had direction finding antennas which use a diodes in the switching circuitry, and the diodes were blown. In those cases, though, I think they had the DF antennas too close to the transmitting antennas.

    The antennas will influence the radiation pattern of each other. But trying to optimize placement to least affect radiation patterns is pretty futile. Keeping them as far apart as possible, but still providing some ground plane around them is the best one can do.

    Mike/K0LPM
     
  5. Tom Stefanac

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    Thanks guys!

    No I don't have an antenna farm on the vehicle, I only have the Wilson 1000 CB whip, 5/8 2m whip and dual band whip. The scanner antenna is a Hustler BNC whip, other than that I only have a small 301 Wilson Cellular Antenna (no amp).

    Would be nice to have all the other stuff but how much of it would I really use? Probably not much! The laptop, GPS and cell phone provide most of the info your eyes cannot.

    But getting back to burning up stuff, I guess the scanner taught me a lesson about spacing and input. The HAM radio gear does not react on TX but the scanners will click on anything above 50 MHz as long as I am TXing above 50 MHz.

    If I key up anywhere on the 2m band it will kill the cell phone data connection. The dual band antenna has its 1/4 wave 2m section between the base and capacitor/coil, the UHF 5/8 section is above that. The Wilson 301 is only about 12 inches tall and sits 1 inch away from the 2m section of the antenna. It will kill the data stream if I use more than 15 watts, but even the full 55 watts will not kill a voice conversation on the phone.


    [​IMG]
     
  6. John Diel

    John Diel EF5

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    I have 4 antenna on a 2000 Explorer. They are about 12 inches apart. Dual band 5/8, VHF 5/8 and 2 cell. No issues on any of them.
     
  7. Tim Stoecklein

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    I'm not sure if you spaced them closely for the photograph or not, but if that's your usual position I'd definitely space them further apart than shown.

    I'd really be interested to hear why the data connection is interrupted if you have the antennas spread further apart than that.
     
  8. David Drummond

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    Be sure to post pictures of the big dents in your car or broken side glass when the first semi you pass going the other way at high speed blows them off ;)

    Seriously though. Stick your ham antenna dead center. Put your CB antenna back on the truck and stick the cell antenna center about halfway between the ham and the windshield.
     
  9. Brad Berry

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    When I worked for a Motorola dealer doing installs, Mother M recommended 18" on center for minimum spacing. I do mine this way, and have had no problem.
     
  10. Andrew Herron

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    No joke, happened to my dad when he was vacationing in the family car. The semi ripped the mag mount off the top and left a huge scratch.
     
  11. mrobinett

    mrobinett EF3

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    Yup, me too, last March near Bartlesville, OK. It was a Wilson Trucker Cell antenna on two 5 inch mag mounts, dump truck passed with about a 25mph cross wind, blew it off, it reached the end of its "tether" and slammed into the left rear quarter panel above the tire, nice dent!
     

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    #11 mrobinett, Jan 31, 2007
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  12. Bart_Comstock

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    I also had the same thing happen to me. To solve the problem I took some zip ties and used them to secure the antennas to my luggage rack on the roof so that they don't move at all. Also my jeep is dedicated solely to chasing so I tend to just not ever remove the antennas and thus it isn't a hassle for me to constantly re-secure them. Of course this isn't an option on all vehicles, but if you use mag mounts and have a vehicle with a rack on the roof might as well save the pain of a nasty dent or scratch or busted window and give the zip ties a try. As far as their spacing on my roof. I have them at opposite corners so they are about 3-4ft apart. I have 3 in total.
    [​IMG]
    Man, just look at that great paint and shiny clear coat! :p
     
    #12 Bart_Comstock, Feb 1, 2007
    Last edited by a moderator: Feb 4, 2007
  13. david diehl

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    WE were just talking on this in a General Class that I am taking.
    You will have a problem with that much Equiptment on a VEhicle.
    You need a switch that will shut off all receivers when you transmit a radio that is pushing that kind of wattage.
    but I will say. That is way too much Radios in a vehicle.
    I only use a Dual band IC-2720, and a Radio shack CB for WEather band, Laptop uses a Cingular wireless air card.
    No Scanner That way I don't hear of all the sherriff-nadoes when I am chasing
     
  14. Shawn Scovel

    Shawn Scovel Guest

    hmm, this is kinda weird for me.. I seem to have problems getting a lip mount to stay on at 70 mph vs the magnet. Actually happened to me the other day when i was on the interstate, i herd a mighty THUD, pulled off and found out my magnet antenna stayed and the 4 allen screws backed off on the lip mount, causing it to fall.

    -Shawn
     
  15. David Drummond

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    Good ol' Motorola NMO mounts.....best of the best!
     
  16. Andrew Pritchard

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    I've got all my antennas mounted on the trunk lid and have never had a single instance with one blowing over. I've got two magmount antennas and my big ham antenna is a trunk lip mount, not one has ever blown off. Never had any problems with them being too close either, each is about a foot apart. (I've got 3 on the trunk)
     
  17. Jesse Risley

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    Get your scanner/receiver antennas as far away from transmitting antennas as possible. You can desensitize your scanner/receiver ("blow" the front end) if you have it on while you are transmitting via ham/two-way radio from an antenna that's simply too close. Be careful.
     
  18. J West

    J West EF3

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    Space all two-way radio antennas at least 1/4w apart from each other, based on the lowest frequency in use. Below is a quick guide for spacing to give you a general idea:

    VHF High (136-174 MHz) ~19 inch spacing
    UHF (430-470 MHz) ~6 inch spacing
    700/800 MHz ~3 inch spacing

    When scanners are used in the same vehicle as two-way gear, get the scanner antennas as far away from the other antennas as you can.

    In the case of CB antennas, other HF antennas and the like...It's impractical to try and maintain 1/4w spacing from other antennas, so get them as far away as you can. In my case, I've got a CB antenna spaced 19" from a VHF 1/4w antenna (hooked to a 110 watt radio) and there is no interaction between the two.

    YMMV.
     
  19. Jesse Risley

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    Here's my setup

    I have a 99 Silverado Z-91 and a 2005 Impala as chase vehicles. You can see that even my antenna patters are a bit close for comfort, so I always turn off the scanner when transmitting just to be safe.
     

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  20. David Drummond

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    That's interesting. I've been blowing out 50watts for years less than 12" from my scanner antenna and have yet to blow one out. It blocks reception when transmitting, but it comes right back when you unkey. Maybe Uniden scanners are just good like that.
     
  21. Hans Schroeder

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    I've had my 45w 2m antenna as close as 12" to my scanner antenna, and have never had issues, save for what David was describing above. The past several years, I've had the scanner antenna on the trunk, and the 2m on the roof though. I have had a boomerang near the back of the roof, and before it was permanent, it did fly off once. You'll have to ask Steve Miller (OK) what a flying boomerang looks like... It did hit the rear window (loud thud), but didn't break it. It also put a nice size dent in my rear quarter panel when it finally came down. :)
     
  22. Jesse Risley

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    It's not that you're going to physically burn up or blow out the scanner, but transmitting too close to a receiving (scanner) antenna could overload your radio and desensitize it across the board....one of the "symptoms" of this problem would be major intermod in certain areas...nothing terriblty serious but extreme cases can render the scanner basically unusable without an attenuator. I would just recommend that you turn the scanner off while transmitting, at least to be on the safe side.

    If your current mode of operations works find and you are satisfied then disregard my suggestions. Some people have no problems..but just beware it can happen if the two antenna are less than 1/4 wave apart at the least.
     
  23. david diehl

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    The rason that the scanner is cutting out when you transmit your 2M radio is because the scanner is getting overloaded with the signal that is coming out of your VHF.
    I am extreamly suprised that you have blown your scanner yet.
     
  24. David Drummond

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    umm, yeah I knew that. Point was that I've done it literally for YEARS with Uniden scanners and haven't blow one out yet. Why, I don't know, but it has never happened.
     
  25. Tom Stefanac

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    In the very first post I mentioned that of all the scanners, only the Uniden 250D (without APCO card) was zapped. The VHF side did not suffer that badly but the UHF side lost all sensitivity. A diode within the scanner between the BNC pin and pre-amp was cooked to crisp.

    The thing is I had keyed up on VHF many times using 50+ watts and never killed the 250D, it was only when I keyed 50 watts on UHF (440 MHz) that I cooked the scanner. Desensing equipment temporarily was not the concern; the main concern is cooking it permanently.

    Front end overload is an obvious problem and issue anytime, even adding an antenna with too much gain in a high noise environment is an issue just like putting an aftermarket 17 inch whip on a portable unit will often increase the front-end and with that noise will become more of an issue.

    Getting back to the scanners after doing some research and making some calls I have discovered there are several Uniden scanners which are prone to being cooked due to the relationship that exists between the BNC feed pin and the diode within the radio before the pre-amp. Most don't have a problem unless you really use a lot of power (100+ watts) but the 250D, 246T, BC350A, BC72XLT, BR330T and BCT8 are the main ones to watch. Most of them are portable and are not built with overload in mind unlike the base/mobile models which more or less are.
     
    #25 Tom Stefanac, Feb 17, 2007
    Last edited by a moderator: Feb 17, 2007

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