RG-8X or RG-58 for mobile install??

Discussion in 'Mobile Radio Systems' started by W9DSD, Sep 13, 2011.

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  1. W1VT

    W1VT Ham Member QRZ Page

    N6YB explains that in the "SWR" mode, the meter actually measures reflected power with an X2 multiplier.

    Thus, if it reads 10 watts(using the power scale), you actually have 5 watts reflected power. He suggests calculating SWR based on the ratio of reverse to forward power.
  2. K7JEM

    K7JEM Ham Member QRZ Page

    Yes, that is the way to do it. The SWR scale is only accurate when the forward power is full scale, which it rarely is, and never in this case. The power scale is better to use, as the calculation can be made better that way.

  3. K7JEM

    K7JEM Ham Member QRZ Page

    Here is another quick test. May require two people. Key a 25 w signal into the antenna, and while watching reflected power, have someone bring their hand toward the bottom part of the antenna rod. Start from about two feet away and slowly move your hand toward the base, while you watch the SWR. If the meter starts going up as the hand approaches, your antenna is too long. If the meter starts going down, and either continues to go down, or hits a dip, then starts increasing, your antenna is too short. If you see no change, your antenna is not radiating.

    Don't touch the antenna! You don't need to get closer than about 3 inches or so.

  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Unless the ground wire is VERY short, it can be introducing problems into the performance of the system. Stake bed installations are notorious for grounding problems.

    Although for lowband installations (46 MHz range), the old Lone Star Gas Company used to install full quarter-wave whips on top of stake bed racks. Unfortunately, they had all sorts of problems especially when the trucks were in motion with breaking up, etc. The "cure" for this was to install a short piece of braid from one of the mounting bolts of the "ball mounts" to the cab of the truck (less than 6 inches which would equate to less than 2 inches for 2-meters). This single "trick" resulted in the company stopping to use their "old" radio service company and giving the service contract to the company that I owned. Several previous contractors had not been able to "solve" the problem! This was back around 1980.

    Glen, K9STH
  5. W9DSD

    W9DSD Ham Member QRZ Page

    Well, thanks for all the tips and advice. It was greatly appreciated. I thought maybe the coax was junk, so i disconnected it and installed a 12 foot jumper from the rig to the antenna. Same exact issue as before. I guess the stake mount is just a s%#@ way to go about it for UHF/VHF, so i will resign myself to the use of mag mounts. I have about 3" if i'm lucky between the roof of my truck and the garage door, so i don't see any other options. A permanent roof mount is out of he question with the clearance i have. Thanks again.
  6. W1VT

    W1VT Ham Member QRZ Page

    Too bad nobody has invented a mobile antenna that will get out of the way of garage doors and other obstacles.
  7. WA7NXI

    WA7NXI Ham Member QRZ Page

    You need to install a permanent NMO mount in the roof of your truck and then use a genuine motorola QW stainless steel whip unity gain antenna....they are extremely tough and can handle getting bent 90 degrees right at the base all the time to clear garage doors, etc. Most police/fire vehicles use these antennas just for that reason, it's what I use on my Nissan truck and it has been beat to holy heck for years and it's still working great....plus they are very inexpensive compared to the fancy Diamond/Comet antennas avail on the market. Good luck.
  8. KF5FEI

    KF5FEI Ham Member QRZ Page

    We use the generic dual-band NMO mount 1/4 wave antennas on roofs of our cars with clearance problems -- works well and they are $12-14 at most hamfests. Palmetto Antennas usually has them, but other vendors sell them too. On the road we swap them out for the cheapy dual-band 5/8 wave antennas for a bit more range.
  9. K0RGR

    K0RGR Premium Subscriber QRZ Page

    It's pretty unlikely on VHF/UHF, but there is a small chance that your rig is 'taking off' and producing a spurious signal somewhere that the antenna does not perform well. It's probably just a coincidence that you're wiping out an FM radio nearby, but it could also mean a spur somewhere near the FM band. Try adding another piece of coax in series with the one you have - make it a bit longer. See if that changes things. It's possible that you just have a peculiar mix of reactances at that particular frequency, with that length of cable.

    I think I agree on the stake bed mount. I would think it would be tough to make this work on VHF/UHF. I've used 3/8 mount antennas on 2 meters but not on UHF.
  10. K4LMP

    K4LMP Ham Member QRZ Page

    I am using a bed stake mount on my dual bander with no problems. I am running about 20 feet of RG-8X coax. For grounding, I ran about 6 feet of #8 wire to an existing point under the truck where a bunch of ground connections are tied to the frame. I used an MFJ antenna analyzer to adjust the antenna. With the antenna as short as possible, the SWR was 1.2 at 146 Mhz, and about 1.8 at 148 MHz, so I left it alone and didn't mess with it anymore. I can get into any repeater in my area. Here is my antenna http://www.jetstream-usa.com/jt7905b.shtml and here is the mount I am using http://bellsouthpwp.net/b/r/breedlove1/page34.html go down and to the left to the Heavy Duty Side Stake Pocket Bracket. $45 and the best you can get. I also used http://bellsouthpwp.net/b/r/breedlove1/page33.html the picture on the top left, with the brass nut. I have no problems and can hold the mount and shake the whole truck with no movement at all (3500 1 ton 4WD crew cab dually with diesel). I highly recommend Breedlove Machine Shop for quality mounts.
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