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Re: [TowerTalk] Antenna shift, VHF/UHF matching unit burning out

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Antenna shift, VHF/UHF matching unit burning out
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
Date: Mon, 21 Apr 2014 09:08:24 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 4/21/2014 6:18 AM, Scott wrote:
My suggestion is throw the Diamond antenna off the tower and put something up worthwhile. Those things are useless imo.


I've found the 21' tall, collinear to work very well as I did the Comet. Both are limited to around 200 watts. Until recently I never had a problem. I have to be careful as there is one repeater that is right at the limit, or even closer than it should be and even though it's not line-of-sight, I hear them often and they use the same PL we do. I did have to replace the Comet when it took a direct lightning strike. The top looked like an exploded cigar. The brass element was melted back 3 or 4 inches. The Diamond replacement has been in the same position for a good 10 years. The tower has taken many strikes, but non to the antenna, half way down.

Simplex, I could work nearly as far (FM) most days as I could with the stacked 12L Yagis. I've had very good luck with their mobile antenna as well. There is one problem with the big mobile, duo band collinear on the SUV though. Because of the height and accentuated rocking, after a few years they become loose and you can hear them rocking in the mount. I have to take the base apart and lap the connection for a tight fit and they are good for a few more years. I can work the regional repeater for about 40 - 50 miles. The quadrant was greater than 60 miles. With the car in the parking lots at MSU I could reliably work the pleasant valley machine on 12/72

I use them with the mag mount. In the winter you make sure it's where you want it, because in cold temps they are there to stay, unless it goes into a heated garage.

73

Roger (K8RI)

Scott
N1CX

At 10:23 PM 4/20/2014, Roger (K8RI) on TT wrote:
I think that I've lost a ground. The SWR has gone sky high on the Diamond 144/440 repeater antenna. As the antenna is only rated at 200 watts the matching network is pretty light duty. Also, the AV640 which originally was very easy to match, is now difficult to find the resonant points.

I believe this is due to a DC bias through the matching transformers putting the cores into, or near saturation.

I keep using, I think and I believe as I really don't know, but it sounds logical.

The new AV840 just arrived and I need to work on the Diamond antenna, so I need to find the problem before going any farther with the antennas.

I had been talking to N8ERF on the repeater when he mentioned I was getting noisy. I immediately stopped transmitting, came in, got the SWR bridge, took it to the shop and put it in the line. The SWR had gone from 1.2:1, to off scale. I think the finals are OK. I certainly hope so.

So tomorrow, I'm going to add one of those grounding plates that tie everything together and to the station ground. The space behind the desk out there is a mass of wire, that could be neatened up, so I can see at a glance what is connected to what.

That and all the projects need to be finished up so I have an organized shop in which to work.

Any thoughts on the grounding would be appreciated

73

Roger (K8RI)

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