[SECC] Coax replacement
Jeff Clarke
ku8e at ku8e.com
Sat Nov 14 10:10:37 EST 2020
I like the RigExpert antenna analyzers. I own a AA-54.
Jeff
On 11/14/2020 10:06 AM, Bill Coleman wrote:
> You know, it may be time for me to think about purchasing another antenna analyzer. I bought my MFJ-259 more than 20 years ago.
>
> The MFJ-259 is still terribly useful, but something more modern that can measure complex impedances, or show return loss directly would be useful.
>
> Suggestions?
>
>> On Nov 12, 2020, at 7:18 PM, Kevan Nason <knason00 at gmail.com> wrote:
>>
>> I used method two. Have a break point at the end of my 175 ft run to the elevated vertical. Rightly or wrongly, I figure the results from that are indicative of what is going on in the feed lines to the tower and inverted-L too. They were all installed at the same time. I was glad Jeff shared his loss for 75 ft. His loss of 1 dB in 20 years (close to that anyway) is consistent with the loss I measured in the 175 ft run adjusted for length. Some other assumptions to make in there, but the specified loss of 213 to Buryflex isn't majorly different so it makes me think what I came up with is probably reasonable. Especially by the time you throw in meter accuracy and manufacturing variables. Jeff is definitely a better tech than I am so I trust his work over my own. Don't forget when you measure return loss to divide it by two to get one way loss. Even though that long feed line is only used on 40 and 80, like Jeff I measure it on 10 meters since that is the worst case loss situation should I decide to put a high band antenna out there someday. Although it could be that since he operates single band on 10 meters so often that he is far more interested in 10 meter loss than I am.
>>
>> Bill, any chance someone near you has an analyzer that measures return loss that you could borrow? I do that every few years when I need something that covers 450 MHz.
>>
>> Kevan N4XL
>>
>> On Thu, Nov 12, 2020 at 6:25 PM Jeff (W4DD) <w4dd09 at gmail.com> wrote:
>> Two ways to measure loss. I've used both.
>> Method 1
>> Terminate coax in 50 ohm load.
>> Set transmitter for desired band and power level, say 100 watts.
>> Measure power into the coax with a wattmeter, measure power out into the DL with the same wattmeter. You may need a helper t key the transmitter for the second measurement.
>> Loss = 10 log (p1/p2)
>>
>>
>> Method 2
>> Short the coax at the far end with a PL259 shorting plug
>> Using something like a MFJ-259, change mode to loss.
>> Readout is in dB.
>>
>> I usually measure at one of the higher bands like 10M since loss will be more apparent there. Compare loss to spec for the cable.
>>
>>
>>
>> -----Original Message-----
>> From: Bill Coleman <aa4lr at arrl.net>
>> Sent: Thursday, November 12, 2020 6:07 PM
>> To: Jeff (W4DD) <w4dd09 at gmail.com>
>> Cc: Kevan Nason <knason00 at gmail.com>; secc <secc at contesting.com>
>> Subject: Re: [SECC] Coax replacement
>>
>> Cats will tend to catch critters even if they are well-fed. They just won’t eat them. I speak from experience….
>>
>> Good info on the coax. My coax run to the A3S is 19 years old, and I replaced the rotator loop as part of my rebuild, but I haven’t yet decided to replace the run up the tower. It wasn’t as exposed as the rotator loop.
>>
>> I’m adding a grounding block at the top of the tower, so I can replace the run to the shack at any time.
>>
>> How do you go about measuring the loss of the coax? What’s the technique?
>>
>>> On Nov 12, 2020, at 9:38 AM, Jeff (W4DD) <w4dd09 at gmail.com> wrote:
>>>
>>> OH no, Kevan hit on one of my hot topics, so I can’t resist adding to his info.
>>>
>>> I too had read that coax should be replaced every 20 years. I’ve had a 100ft piece of RG-213 in a black 4 inch drainage pipe to the tower for 20 years. I test the loss every few years, but it actually has not gone up much, many half a dB at 28MHz.
>>>
>>> What I have found is coax goes bad primarily because various varmints like to chew on it. The biggest offenders, at least here north of Atlanta, are squirrels. It seems they like chewing on it right down to the braid. Once that happens, water infiltrates the braid and wicks along 10 to 30 ft. The copper braid corrodes and the coax gets real stiff. Losses seem to go up then. The squirrels not only like coax, but also like rotor cables and control cables.
>>>
>>> I’ve found two solutions:
>>> My wife has kinda adopted a feral cat (now spayed and living a good life). That cat is she still is working some. My wife says I owe her for the good job she did.
>>> Second solution, where critters are likely to chew on the cables (like near the base of the tower) I cover the coax with .5 inch split loom tubing. You can get 100ft very
>>>
>>> I went to the Orlando hamfest this year and did run across a hundred ft of almost new 50 ohm ½ inch Superflex (like corrugated hardline but more flexible). The commercial vendor didn’t seem to want to haul it to another hamfest, so offered me both a 75ft piece and a 100ft piece for the price of a 75. One of those deals that you are not looking for but can’t pass up. I installed the 75ft piece to the tower base with a short flexible jumper at the house end and saw about a 1dB decrease in loss at 28MHz over the old coax. In addition, it is a solid outside conductor so even it the critters chew through the jacket, no water will wick along the shield. The other piece of Superflex may be used to feed a 6M beam one of these days, with an appropriate flexible jumper near the rotor.
>>>
>>> My other project is to get a Tic Ring Rotor working and mount it on the tower at 50ft to turn the A-4 tribander. That will allow me to rotate something for 15 and 20M. but as Kevin said, we don’t move as fast as we used to.
>>>
>>> And am working on several power line noise issues to the NE and SE. I’ve identified the defective hardware on 5 of the 7 poles, and am awaiting the engineer and line crew. Those noise sources killed me during the CQWW a few weeks ago. Hard to hear weak EU through S-4 noise.
>>>
>>> Jeff
>>>
>>>
>>> From: SECC <secc-bounces at contesting.com> On Behalf Of Kevan Nason
>>> Sent: Thursday, November 12, 2020 7:36 AM
>>> To: secc <secc at contesting.com>
>>> Subject: [SECC] Coax replacement
>>>
>>> I like Jeff's, W4DD comment about posting at least once a month.
>>>
>>> Here's something I put in another clubs newsletter about current activity at N4XL:
>>>
>>> · Truth or fiction? It came to my attention while preparing a presentation on different types of feedlines for our local club that most coax cables, including the non-contaminating type, have a finite lifetime of between 15 and 25 years. After that, the loss begins to creep up. I didn’t care enough when I first set up this station to record station data so have no idea what SWR or reflected loss changes have occurred over the years. It seems likely any changes due to aging coax would be waaayyy to slow for me to notice over the 20 years that have passed since I installed my coax runs; and to that point I can’t say I actually do have excessive loss. So, I started looking on the internet. Most posts say it should be replaced. I ran across a discussion about this on the TowerTalk reflector (at least I think it was that one) where K3LR (I think) said he doesn’t replace his unless he sees a problem. He has quite a station so maybe it doesn’t need to be replaced.
>>>
>>>
>>> Looking at my exposed coax it is rough feeling and no longer shiny and black. For the most part, it is Davis Buryflex. I bought some new Buryflex for a doublet I installed a couple years ago. The 20-year-old cable seems to be more flexible than the new. Also, when I put the Rigexpert antenna analyzer on the 175 ft run out to the elevated vertical it says the 7 MHz return loss is a bit over 1 dB (one way) higher than the specifications say it should be. Since most coax runs were installed at the same time I assume they all would read similar loss.
>>>
>>>
>>>
>>> It is getting harder to work as these bones get older. About 200 ft of my coax is
>>>
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>> Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net
>> Web: http://boringhamradiopart.blogspot.com
>> Quote: "Not within a thousand years will man ever fly!"
>> -- Wilbur Wright, 1901
>>
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> Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net
> Web: http://boringhamradiopart.blogspot.com
> Quote: "Not within a thousand years will man ever fly!"
> -- Wilbur Wright, 1901
>
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