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   Home   Help Search  
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Author Topic: High SWR.  (Read 692 times)

Posts: 4

« on: June 21, 2001, 08:50:28 PM »

I just got done installing a complete Larsen NMO mount kit with a NMOQ2/70B 1/4 wave antenna.  I drilled the hole in the center of the roof on a S-10 extended cab pickup.  My readings across the 2M band are a consistent 2.1:1.  Everything is grounded really good.  The only thing I did that was a little different was I cut 24" off the end of the cable before I solder the end on.  I have tried two different radios and Larsen was kind enough to send me another antenna just in case there was a problem with the first.  Same results.  The SWR on my other vehicles are great.  So I don't think that my watt meter is bad.

Please Help!

Posts: 1

« Reply #1 on: June 21, 2001, 10:44:51 PM »

I'd be suspicious of the cable -- a crimp that's not quite creating a short or something like that.  Since the only "mod" you made was to cut the cable, I'd take a look there.  Just the removal of 24" of cable shouldn't be a problem itself unless there was some hidden damage done to the cable.

Also, is the SWR high regardless of frequency within the band?  Could be a resonance thing -- I'm not familiar with that particular antenna and so don't know if needs trimming to length.


Posts: 75

« Reply #2 on: June 22, 2001, 11:41:32 AM »

You might want to double check the contact between the NMO mount and the roof sheet metal if you've not already done so. I was unable to get decent metal-to-metal contact until I ground the paint completely off around the mount hole (inside and out) with a Dremel tool. FWIW, SWR across the 2m band with my Larsen setup runs 1.2:1 to 1.4:1. Mighty fine little antenna systems.

Posts: 17411

« Reply #3 on: June 22, 2001, 11:44:12 AM »

I presume you have tried adjusting the antenna length. and made
sure that the antenna mount is properly grounded to the metal
skin of the truck.  My next guess is that there is an open
connection on the coax shield somewhere, likely at the antenna.
Another possibility is that there is another antenna (or perhaps a
camper shell, pipe rack, or other metal object) in the near field of
the antenna which is detuning it.

And quite possibly you have already checked these things - it is
hard to know at this distance!

One other possible cause would be if your transmitter has some
spurious output, for example, the second harmonic, at a frequency
where the antenna is not well matched:  in that case, you probably
will never be able to see a 1 : 1 match on a resonant antenna (but
it will look fine on a dummy load, which is not frequency selective.)

Posts: 1

« Reply #4 on: June 24, 2001, 02:11:05 PM »

You might try and find a local communications shop--and see if they have the adaptor ( I have one) which allows you to adapt the nmo mount to an so239 (UHF connector).  This allows you to use other antenns, or more important here, allows you to attatch a dummy load at the antenna connection.  ALSO, are you SURE your swr/wattmeter is accurate at VHF?   Might try another meter.   Any chance you are just plain cutting the antenna way too short or too long?  

Posts: 1435

« Reply #5 on: June 25, 2001, 05:23:55 AM »

If I remember correctly, there isn't any gain by going from 2:1 to 1.5:1.  

So, if you don't enjoy the detective work, why do it?
(I'd have to play with it!)

You can clip an alligator clip, with or without wire on the other end of the jaws, on the end of the antenna to see if the ant is too short, or to see if you can find a dip in SWR.

Posts: 4

« Reply #6 on: June 25, 2001, 09:56:33 AM »

What I tried this weekend.

1) I have two brand new NMO Larsen antennas.  They act no different.  SWR on both is around 2.5 thoughtout the entire 2M band.
2) Put on a Diamond 770 1/2 wave.  Worked great. SWR was about 1.2 or less.
3) Made a 24" patch cable to get back the 24" I cut off.  No change.
4) Added temp. ground straps to the base to get a better ground.  No change.
5) Used a different SWR meter. Same results.
6) Had someone hook up their antenna analyzer and see the antenna is resonating around 153.000.  I tried to adjust it but no luck.

I am at a total loss.  I would love to use a 1/2 wave all the time but the reason I did all this was to have a enough clearence for the garage.

If anyone has any other suggestions let me know.

Posts: 17411

« Reply #7 on: June 25, 2001, 12:53:38 PM »

Sounds like the antenna is too short.  Are you sure there isn't a
set screw in the base which you can loosen and pull the whip out
a bit longer?  Most mobile whips I've seen have some sort of
adjustment.  You only need about 3/4" more length.

Otherwise, the alligator clip on the end of the antenna should bring
it in just about right for 2m.  

Posts: 4

« Reply #8 on: June 26, 2001, 08:12:48 AM »

Yes, it does have a set screw and even with it fully extended it still does not come down low enough. Sad

Posts: 2

« Reply #9 on: August 21, 2004, 08:11:02 PM »

Interestingly enough I am having the same problem with this antenna with my mobile mount Yaesu FT-8800R. Mounting it on a Ford Taurus using Comet adjustable lip mount. I have run a ground wire (base of antenna shows good ground with VOM), scraped paint, lengthened the rod as far as I can. It is electrically short with best SWR in upper part of both bands and worse at lower.

I am using an MFJ 862 SWR meter.

I brought the meter back indoors and connected the FT 8800R (antenna is GP-9)and the ranges are normal so that decreases chance of this being a harmonic albeit that was a good thought.

??Bad antenna design? Perhaps but I had a unity gain dual band antenna that gave the same readings so it is more likely something to do with the mounting.
I am curious that if the antenna analyzer read this as harmonic at 153 MHz that Larsen may have a batch of 270's with incorrect rod length. When I pulled it out the 1/4" it would go the SWR did improve minimally.

I appreciate any feedback that might be useful. This one has me stumped.
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