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Here is where you should mount your CB and HAM antennas on your Jeep

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Old 04-12-2017, 07:00 AM
  #301
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Default This is a good thread re: SWR, but ...

There is more to antenna placement than SWR values. Yes, SWR will measure how well you have matched ("tuned") your antenna to your feed line for a given installation, but it doesn't tell the whole picture. We also need to consider the resonant frequency of the antenna, and what frequencies the SWR was measured at. What good is it to have an awesome SWR for an antenna that is less than optimal for its intended frequencies?

Case in point, our Larsen NMO 2/70 mounted to a JK's front fender. Our antenna shows an SWR of 1.0 over a wide range at this location. Awesome, right? Yes, except for this location reacts with the hood sheet metal slightly above the antenna's base, and with the A-pillar. As a result, that low SWR occurs at 142 MHz, which is below the 2 meter band plan. Also, this antenna happens to be resonant at around 142 MHz as well (no surprise, since SWR often corresponds to resonant frequency with major brand antennas such as Larsen, Diamond, Tram, etc.). Unfortunately, most of my area repeaters are 147 MHz or above, and our antenna's SWR is 2.0 at that frequency. At least the bottom of the 2 meter band has a relatively low SWR. See graph for details.

For the record, we are going to keep the location in this place, and trim the antenna to bring the SWR and resonance within the 2 meter band. Unfortunately this will impact 70cm performance as well, but we'll keep a spare OEM element for those times that we want to transmit on both UHF and VHF.

One more thing. Our CB antenna is an evil between the tire carrier and tailgate install, but we have SWR of 1.0 on channel 20 & 1.4 on channels 1 & 40. Antenna is resonant on 20, and less so on the other channels. It's possible to have a good installation at that location if the rest of your install is solid (e.g. GOOD BONDING), but the radiation pattern will definitely suffer.

Anyway, I thought I would post this just as a caveat for those focused solely on SWR. SWR is important, but we also need to focus on the FREQUENCY at which that SWR occurs, and the resonant frequency of our antenna, when determining a final installation.

Radios: ICOM ID-5100a, Cobra 29 NW WX BT
Antennas: Larsen NMO 2/70, 3' FireStik FS-2
Mounts: Accessories Unlimited AUJ1 & Laird MBOX for ham, Breedlove #203 disconnect and Cooltech gate mount for CB.
Coax: 9' of LMR-240 UltraFlex for ham, 18' FireStik FireRing for CB

-K4GMO (Dawn's husband)


Radios:

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Fender Mount SWR and Capacitance chart for 2 meters:

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Fender mount ham antenna:

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CB installation:

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Breedlove #203, 3' Firestik:

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Bonding with Electric Motion Company EM2081 (before the Breedlove #203). Ground straps are bolted to nutserts, which are then crimped to steel plates that are riveted to exposed sheet metal (everything subsequently sealed with epoxy and painted with factory paint to prevent corrosion):

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The analyzer on channels 1, 20, and 40:

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Last edited by OnBelay; 04-12-2017 at 07:12 AM.
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Old 01-31-2018, 10:44 AM
  #302
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Default CB Antenna mount outside spare tire

So, I've tried searching for this without success. I am planning on mounting my cb antenna to a custom bracket outside of the spare tire. It would be similar, in concept, to the Vector Offroad Exo Rack, linked below. Will this help solve some of the issues of mounting the antenna at the rear of the vehicle? My CB is a midland 75-822 if that matters
https://vectoroffroad.com/products/exo-rack
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Old 02-01-2018, 08:27 AM
  #303
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Default

If the antenna is mounted on the outside then yes because at least one side of the antenna is exposed. The issue is when its mounted between two plates of metal, that is bad because the signal will bounce between the metal instead of away from the antenna.

Hope that helps
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