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RE: What do you recommend for FM transmitter antennas?
To: "'CRTech'" <crtech@crtech.org>
Subject: RE: What do you recommend for FM transmitter antennas?
From: "Bill Hurne" <billhurne@pilgrimradio.com>
Date: Wed, 4 Apr 2018 09:20:42 -0700
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Our two bay Shively 6812 coax harness failed after only 5 years on a brutal mountain top that receives much ice and winds well over 100 mph.   The coax had pulled loose out of the connectors.   The antenna itself survived which is inside radomes.   We got back on the air temporarily by connecting a single bay without the bad harness, but the SWR was elevated because a single bay feed impedance is 100 ohms instead of 50 according to Shively.

Bill

-----Original Message-----
From: Andy Larsen [mailto:andy@whwl.net] 
Sent: Wednesday, April 04, 2018 5:55 AM
To: crtech@crtech.org
Subject: Re: [CRTech] What do you recommend for FM transmitter antennas?

Ditto to this. Our main station has a 12-bay ERI. All of our Class-A stations and translators use Shively antennas. Most are the 6812 one or two-bay model. The only problem we have had was the coax between the bays wore through and failed after over 20 years of service. When it was replaced, I believe the piece that was cut was the wrong length; because the coax length is critical to the tuning, we couldn't get acceptable SWR--same issue we were having with the worn piece, which led us to believe it was an antenna problem. We replaced it with the Versa2une, which  has been performing extremely well, with imperceptible reflected power.


More info on our 6812 coax problem for anyone interested in continuing to read...could be useful if you ever have to replace the coax. :)

So the configuration of the 6812 (ours anyway) is a bottom-fed antenna, and the inter-connect between the two bays is a piece of coax cut to a specific frequency-dependent length. The length and the method of wrap of the excess coax around the mast are integral to the tuning of the antenna. The early multi-bay antennas had 10-inch stubs feeding each bay off a "T" connector, including the top bay, but replacing the T with a barrel connector for the top bay. This allowed all the inner-connecting coax to be the same length. Later models just cut a longer piece for the top bay, eliminating the barrel and the jumper. When we had the replacement coax cut, we were looking at a later blueprint, and cut the coax to length and sent it up the tower. We could never get full power with acceptable reflected power, so ordered a replacement antenna, believing that the problem was in the antenna. Once the antenna came down, I discovered that the climber had replaced the coax, but left the ten-inch stub in place attached to the top bay, while we didn't realize that the stub was there. This made the coax 10" too long for our frequency. I suspect the antenna was probably actually good, and that had we had the correct length of coax (eliminating the extra 10" in-line
jumper) it would have worked just fine. I still have the antenna, but have not had the need to test this. Either way, the antenna worked beautifully for over 20 years before the coax failed.

Andy


On 4/3/2018 1:17 PM, Pat Wahl wrote:
> Lots of good brands out there. We use ERI and Shively. Lately, we have 
> been installing the "Versa2une" by Shively. Comes in a kit, quality 
> heavy stainless, easy to put together and you tune it yourself to any 
> frequency without expensive test gear. That way, the antenna can be 
> re-utilized in the future without major cutting at the factory. Our 
> coverage experience from this antenna has been phenomenal. We have 
> used both single and 2-bay configurations.
>
> Pat Wahl
> Chippewa Falls, WI


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References: What do you recommend for FM transmitter antennas?
(Dave Land <dave@klvv.com>, 3 Apr 2018 16:26:56 -0000)
Re: What do you recommend for FM transmitter antennas?
(Pat Wahl <pwahl@wwib.com>, 3 Apr 2018 17:17:56 -0000)
Re: What do you recommend for FM transmitter antennas?
(Andy Larsen <andy@whwl.net>, 4 Apr 2018 12:54:51 -0000)
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