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Author Topic: 2m/70cm mobile antenna measurements - Not feeling good about them  (Read 2601 times)
W7TJD
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Posts: 3




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« on: July 23, 2013, 11:23:09 AM »

Hi all. I'm looking for some piece of mind on my 2m/70cm mobile install. My background is in commercial cell sites, and the only amateur band system I've tested was my old 2m setup in my last Jeep.

I have a new Jeep with a TM-V71a and a Larsen 2/70 mounted on the drivers side fender on a NMO drill-in lip mount. Since there are a lot of tight tree-lined trails here in the PNW I installed the Larsen spring between the antenna element and the coil. I had to shorten the antenna with the spring in there, and looked for the best balance between the bands to determine how much to chop off.

The VSWR is not that hot on either band, but it's the Return Loss and Cable Loss measurements that are bugging me. The cable loss is through the roof on 144.

Cable Loss
144MHz 6.39dB
148MHz 4.61dB
440MHz 4.40dB
450MHz 4.69dB

Return Loss
144MHz  -12.78dB
148MHz   -9.21dB
440MHz   -8.80dB
450MHz   -9.37dB

VSWR
144MHz  1.60
148MHz  2.06
440MHz  2.14
450MHz  2.03








All these tests were performed on an Anritsu S331E I borrowed for a few minutes....

Bad UHF connector? I do suck at installing those...

Thanks in advance...

W7TJD
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N6AJR
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Posts: 9927




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« Reply #1 on: July 23, 2013, 12:09:07 PM »

looks ok to me, you will have a little loss with the coax used on mobile mounts, but any thing with swr under about 2:1 will be fine on 2m/440. 
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W7TJD
Member

Posts: 3




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« Reply #2 on: July 23, 2013, 12:50:04 PM »

Thanks N6AJR. It's the cable loss thats really killing me. 5.5dB average on 2m is pretty darn high for a 15' run of RG58. The cable should only have about .93dB of loss so either the UHF connector or the NMO mount is killing the rest. I may try replacing the connector and retesting to see if I get better results.
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M6GOM
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Posts: 1012




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« Reply #3 on: July 23, 2013, 06:02:34 PM »

Something is amiss.

If there was a fault you'd see the losses on 70cm higher than 2m. My guess is there may be a fault with the test equipment, patch leads or you not knowing how to use it correctly.

Cable loss being higher on a lower frequency makes no sense.
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K1CJS
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Posts: 6061




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« Reply #4 on: July 24, 2013, 05:18:45 AM »

How did you determine that it's cable loss that's throwing the measurements off?  If I were you, I would look at the spring that you put on the antenna base before you jump to that conclusion.  Springs are notorious for sending antenna measurements off, especially if that spring did not have a braided 'strap' inside to connect the top and bottom of the spring mount points together.

It's hard to see that strap sometimes--or to be sure that it's got good connections to the top and bottom of the spring, so a simple test would be to put a wire jumper together and bridge from the top of the spring to the bottom.  Either that or remove the spring temporarily.  You can always make a short antenna extension or a n alligator clip to compensate.  If you see a major difference, (improvement) you would know then what you could do to improve your antenna system--get rid of the !%@#?&!! spring!   Roll Eyes  73!
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K7RBW
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Posts: 398




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« Reply #5 on: July 24, 2013, 05:46:39 AM »

I'm in the "something's fishy" camp on this one.

RE: cable loss, was that test done with the antenna attached? I thought cable-loss tests had to be done into an open or a short. How can the tester tell what it's seeing is the result of the cable or the load at the end of the cable, if not? In any case, the 440 loss should be about twice as much as the 2-meter loss, and neither should be very high with only 15' of cable.

RE: putting a spring in the antenna, I know the PNW trails well so I know the trees you're talking about. It looks like you did a decent job accommodating the antenna length, but I don't know if it's really necessary. The Larson antennas are pretty durable and stand up to considerable abuse. On the trail, you're probably not exactly DX-ing, so you might consider 2 antennas: 1 stubby antenna for the trail and a longer one (the 2/70) for the road or trails that are less tree-lined. I have a 12" antenna on the roof of my RAV4 and it seems to do almost as good a job as an antenna 3x as long in most cases. More precisely, the antenna that's 3x as long doesn't pull in 3x more signal or distance.

But, if you have a 2:1 VSWR or less as measured at the radio end of the cable, I'd call it good and go wheelin'.
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W7TJD
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Posts: 3




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« Reply #6 on: July 24, 2013, 05:57:49 AM »

I'm in the "something's fishy" camp on this one.

RE: cable loss, was that test done with the antenna attached? I thought cable-loss tests had to be done into an open or a short. How can the tester tell what it's seeing is the result of the cable or the load at the end of the cable, if not? In any case, the 440 loss should be about twice as much as the 2-meter loss, and neither should be very high with only 15' of cable.

RE: putting a spring in the antenna, I know the PNW trails well so I know the trees you're talking about. It looks like you did a decent job accommodating the antenna length, but I don't know if it's really necessary. The Larson antennas are pretty durable and stand up to considerable abuse. On the trail, you're probably not exactly DX-ing, so you might consider 2 antennas: 1 stubby antenna for the trail and a longer one (the 2/70) for the road or trails that are less tree-lined. I have a 12" antenna on the roof of my RAV4 and it seems to do almost as good a job as an antenna 3x as long in most cases. More precisely, the antenna that's 3x as long doesn't pull in 3x more signal or distance.

But, if you have a 2:1 VSWR or less as measured at the radio end of the cable, I'd call it good and go wheelin'.

Ya you're right, CL needs to be done into a short or open. Sad that I forgot that, guess I've been in the office too long.

M6GOM, we do indeed have some "operator error" going on here.

So my cable loss traces are crap, but the VSWR and Return Loss sweeps should be valid.

A short antenna for the woods may not be a bad idea. I was hoping for a set it and forget it sort of setup. I had the spring from my NMO150 setup on my last jeep so thought I'd use it. Of course my 2/70 is too short now without a spring. Smiley

I'll check out my spring and see if its causing issues too.

Thanks for the feedback everyone, we'll see what I find.

« Last Edit: July 24, 2013, 06:08:12 AM by W7TJD » Logged
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