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Author Topic: Properly assembly for a Larsen 270NMO antenna  (Read 5848 times)

Posts: 2

« Reply #15 on: January 26, 2017, 11:54:54 AM »

I personally have a Diamond SX-400 SWR, power meter which covers from 140-525MHz it does SWR and power in 3 scales from 0-200Watts. It can also do average and peak reading. It is a good little meter, and sells new for about $150. For an analyzer I prefer the Comet CAA-500MARK II which is around $400.00 it covers 1.8-500MHz, but it is well worth it. I know that MFJ has their 259, but I tend to stay away from their products due to poor quality control. I know I probably rattled some cages with my statement about MFJ but this comes from my personal experience with their products over that last almost 35 years of hamming. nuf-said!! Or the good ole standard and the best thing out there in my opinion a Bird watt meter. Most of the time I use the Bird to check an antenna.


Posts: 27

« Reply #16 on: January 26, 2017, 05:53:57 PM »

So I found a few spousal-approved minutes this evening. It's pretty cold and windy out and I didn't want to be snaking jumpers all over the roof of my car, so I did the stuff I could do from the drivers' seat with the door closed.


Between PL-259 center pin and outer shield: open circuit
Between PL-259 center pin and cigarette lighter housing: open circuit
Between PL-259 outer shield and cigarette lighter housing: 1.2 ohms (note that today, my meter reads 1.1 ohms lead-to-lead).

So this was pretty much as expected. Nevertheless, still suspecting that the PL-259 was the problem, I cut it off and redid it according to the instructions found here:

Which I have to say is a pain in the butt. But I got it back together, and took new readings, and they were exactly the same.

That being said, I've had enough experience with low-signal electronics to know that a solder connection can read low resistance but still not be solid. Real-world results with the radio on were hard to really determine: local repeaters were clear, distant repeaters were scratchy. I couldn't find anyone to talk to, so I had to judge just from the CW ID reports off the repeaters, which I found inconclusive. I'll post again tomorrow assuming I can get someone to answer a call.
« Last Edit: January 26, 2017, 05:57:44 PM by KC3AYG » Logged

Posts: 27

« Reply #17 on: January 26, 2017, 06:01:35 PM »

"Then the heavens opened, and down came a Paladin CrimpAll 8000, and some die inserts for various sized coax.  Love it, works the first time, every time.  "

Once upon a time I worked in a lab with a complete set of crimp tools. I miss those...

Posts: 27

« Reply #18 on: January 26, 2017, 06:16:36 PM »

What insert is that, exactly?

Posts: 687

« Reply #19 on: January 26, 2017, 06:26:25 PM »

The important thing is not to get too stressed out.

And don't get in a hurry

Remember, this is a learning experience.

When you DO find the issue, you will not only have a sense of accomplishment, you will always remember for next time.

And you don't need analyzers or expensive swr bridges but a little meter like the MFJ 844 would be a nice tool to have
Right now


Posts: 27

« Reply #20 on: January 27, 2017, 06:03:59 PM »

Well, I have a little news...

Dug out my other radio and swapped it in for the BTech. I couldn't hear any difference, although it isn't a perfect comparison as the BTech is dual band with commercial FM receive, whereas the Icom is 2m only. But if there was a difference it wasn't a big one.

I had a idea as I was driving home that maybe I had secured the coax wrong - I wire tied it to a bundle that was already running through the dash, and I thought maybe that wire bundle had high frequency signals on it that was putting noise on the coax. So I pulled over and cut the wire tie, and then stuffed the coax loosely up under the dash. At the point I did this I was attempting to hit a big repeater about 35 miles to my south, with little success - the receive was very intermittently opening the squelch, and when I did get a signal it was almost uncopyable. But as I drove home - in a direction away from the repeater, but with a slight gain in altitude - the receive quality gradually got better, to the point that as I was pulling into my driveway, 50 miles from the repeater, I was able to have a decent conversation with the ham that runs it. Now, I'm still not convinced things are perfect, because he indicated that I was being received at almost full quieting, whereas I certainly was not close to full quieting on my end. I think I should be surprised to be getting into a repeater with better signal than I receive the repeater, yes?

On the other hand, I was actually getting into and received from a repeater that was fifty miles away, on 25 watts of transmit power. So maybe my issue is more with unrealistic expectations than with my antenna?

Nevertheless: continuing my trouble shooting I decided to just double check all of the connections. I took the whip out of the base, tightened down the set screws, tightened the base down to the NMO, screwed the whip adapter into the base with a wrench. Removed the PL-259 from the radio, tightening back down again.

End result: prior to all the fiddling, radio in Monitor mode, I was getting six bars of signal strength just from the static. Afterwards, I'm down to two to four bars, and usually two.

On the other hand, the commercial FM broadcast reception is still poor.

So I don't know what to think. Tomorrow I will do the rest of the continuity checks, and if they check out I think I'm probably going to declare the issue solved, or maybe that it wasn't early an issue all along, and I just need to get more experience before I judge.
« Last Edit: January 27, 2017, 06:16:38 PM by KC3AYG » Logged

Posts: 112

« Reply #21 on: January 27, 2017, 09:00:58 PM »

Sounds like you are on the right track. 

Yes, it could be possible to get into a repeater better than you can hear it - for example, if they use two antennas the receive antenna could be higher than the transmit antenna for whatever reason.  That may not be the case here but I'm just saying it's possible for that to be a situation. 

But 50 miles is a good distance for mobile ops.  Sure, some people get further but there are lots of factors involved on both ends (the radio setup, the repeater, terrain, conditions, etc.). 

At least you did see a change, made some progress, and perhaps have some more motivation to check further and see what you determine. 

As always, keep us posted on the progress and best of luck!


Posts: 571

« Reply #22 on: January 31, 2017, 01:47:57 PM »

Check the NMO tab on the antenna. Bend it out a little bit (Not so much force as to break it.) This will ensure a good solid connection between the tab (which is the center conductor to the whip) and the mount's button.

Failing that not sure if there is a bad coax or if you are grounding it out anywhere.

Posts: 27

« Reply #23 on: February 03, 2017, 04:41:32 PM »

Did that. Thanks for the suggestion.

At this point, I'm not sure there's anything else wrong with the install. I found out that the repeater I was trying to get into has remotes - which receive but don't transmit. So apparently, with this repeater, it is not uncommon to be able to get into the receiver better than you can hear it. All of the other readings on the antenna check out.

I'm saving up for an antenna analyzer, just because I plan to build an HF antenna or two and it seems like a good thing to have. When I get it I'll use it on the mobile, just to see.

But the commercial FM receive still stinks. And I don't understand why. That bugs me.
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