Sample lines: curiosity question

AM Radio discussion. Directional arrays are FUN!
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w9wi
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Sample lines: curiosity question

Post by w9wi » Mon Aug 26, 2019 8:27 pm

So, I'm a TV guy & probably missing something dumb here but...

Why do AM sample lines use such heavy-duty cable? I see discussion in the current thread of using LDF4-50, which is not exactly cheap:) Surely there isn't enough power in the sample lines to require anything that large? Wouldn't something cheaper like Belden 9100 do the trick? Or are these smaller cables just not consistent enough in loss & phase characteristics to be safe for this service?

Curious minds want to know:)
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Deep Thought
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Re: Sample lines: curiosity question

Post by Deep Thought » Mon Aug 26, 2019 11:28 pm

A lot of old (>40 years) systems used RG-8 which has issues with temperature stability and is not really an FCC favorite. When the Commission standardized on a "type approved sample system" they specified solid outer conductor (shield) cable which can be phase stabilized by heating for a period of time. That became the gold standard which continues today. See §73.68.

Over the years I've seen systems built with 1/4" and 3/8" line which were a disaster (1/4") and problematic (3/8") due to things like frost heave and other environmental effects which twisted or outright ripped apart smaller line when it is buried. Even 1/2" has issues with that in some parts of the country.

The FCC will let you use whatever you want for a traditional measurement proof if you can prove that it can meet the stability requirements, but that means you are taking antenna monitor and monitor point readings every day for 30 days for each mode of operation. Method of Moments proofs require solid outer conductor coax of 1° or better measured equal lengths.

As a general rule, 1/2" is the standard now, which is a good compromise between durability, accuracy and cost.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

w9wi
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Re: Sample lines: curiosity question

Post by w9wi » Tue Aug 27, 2019 5:01 am

Makes sense, thanks!
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Re: Sample lines: curiosity question

Post by kkiddkkidd » Tue Aug 27, 2019 5:52 am

And then there was the large Iowa array that someone thought it would be OK to convert the 3/8in unequal length sample lines to equal with various lonnnnnmg lengths of RG8. Some of the RG8 was inside, some outside... It didn't work out very well long term.

I have only worked on a couple of arrays that used 1/4in and "disaster" would have been fairly mild.
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Re: Sample lines: curiosity question

Post by radio_guru » Tue Aug 27, 2019 10:31 am

Having done a few AM arrays from scratch myself, including the 2nd licensed MoM array in 2010, I vowed to never repeat the mistakes of others who cheaped out.

Beyond the physical durability of direct burial rated line is ultimate reached phase stability over time and across various temperature ranges. It's not that well known, but the most common foam annular line when bought new is simply assembled and spooled. It is not "aged" and does change over time even in stable conditions as the foam "finalizes" it's curing. Doesn't matter the manufacturer. The effect is reduced where the spool of line is outside and very cold times. Ma-nature's way of conditioning the line.

Andrew offered and RFS/Cablewave (IIRC) offers both phase stable and temperature cycled products in addition to standard lines. IIRD, Lupen does so as well. The former was/is the most stabilized of the lines as they purposely temperature cycled the lines through a series of hot/cold cycles to wean out early in life aging artifacts, then cut them to precise phase and re-test each length. It is also most costly for obvious reasons. But if you bought and installed that line properly, phase variations were nearly imperceptible over time. This type of line was critical at microwave where any physical or electrical length changes resulted in significant phase changes.

The temperature cycled line was just that... They cycled the line in bulk through a few thermal cycles to accelerate the aging stabilization process which achieved the necessary stability and consistency for AM sample systems. Remember, it's the sample system which is licensed in a MoM array. And it must be stable. Drive lines, OTOH, can be anything as phase/ratio can be compensated for in the tuning. You can't with the sample system.

More over, I typically install the temperature compensated only line and various other cables in fiber duct or at minimum schedule 40 PVC for everything. Except strap and that works amazing well at protecting the lines (sample, tower lighting, ATU power, and drive if it will fit. Never mind I can wait until the project end to pull cable to each ATU.

We tried 3/8" last time and found while it works equally as well at about 1/2" and nearly as durable, the price as 1/2-2/3 of LDF-4, it is a bit more to buy the associated N or DIN connectors, let alone find them.

Then there is building the 36" jumpers made from RG233 or RG142/141 for looping a couple turns of line thru ferite cores and secure enroute to the antenna monitor. Thus taking stress off the antenna monitor connector.

There is much more too... ,..but will save that for later posts on the subject.

Since Compscope bought Andres about 10 years ago, they have decimated the LDF line by discontinuing the LDF-4, 5 and 7 foam lines for cheaper AVA stuff. So if you want Andrew sample line, its's 3/8 only. The rest you need to source separately...like Cablewawe.

And don't forget to place any order for phase stabilized/temp compensated line months in advance as you need to precisely spec the length wanted.

RG

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