[TowerTalk] which type of coax?

Roger (sub1) sub1 at rogerhalstead.com
Fri Nov 19 16:10:11 PST 2010


On 11/18/2010 12:13 AM, Jim Brown wrote:
> On 11/17/2010 12:18 PM, Bill NY9H wrote:
>> The ONLY problem with Belden is that they are setup to ONLY go thru
>> multiple levels of distribution....causing multiple levels of profit
>> margins,,, and unnecessarily high prices....
>>
>> the Davis folks and a FEW others buy&/or have built quality
>> product,,,,and can profitably sell it for 1/2 to 2/3s the price  with
>> NO cut in quality....
>>
>> THE BELDEN STUFF IS VERY GOOD....NO PROBLEM THERE..
> EXACTLY the problem.
>
> For advice on coax, see
>
> http://audiosystemsgroup.com/Coax-Stubs.pdf
Very nice addition Jim.  At least I think it's an addition as I didn't 
see it when reading the tutorial a year or so back, which could speak 
more to my attention span than contents.

There  are several things I'd like to add.
You did a great job of covering the electrical characteristics of the 
cables however there are some physical characteristics of cables that 
could be important.

CNT 240 and I assume LMR240 ( I have a lot of experience with the CNT240 
but none with LMR240) can be a bear to install connectors and get a good 
shield connection.  The cable is quite stiff and the wires making up the 
braid are tiny and VERY *fragile*.  I used to use CNT240 to feed my 40 
meter slopers as they are located in an area where the wind funnels 
through the yard and between buildings making a coax with a low cross 
sectional area desirable.  CNT240 does not tolerate a lot of movement 
and flexing well. I was having to replace the connector at the antenna 
end of the coax every few months and I had them reinforced.  I have also 
found the jacket on the LMR Ultra Flex, or UF versions to be fragile, 
sensitive to UV, and difficult to use standard cable strippers on.  OTOH 
the old "Box Cutter" stripper works very well on the rubber like jacket. 
That jacket being rubber like also tends to stick to or catch on  metal 
where it slides over edges. IIRC it's lifetime rating is about half that 
of the standard LMR cables, but it is the most flexible cable for a 
given size that I've used.

   As you already mentioned, the larger cables are better for station 
interconnections rather than the small ones. OTOH RG-8X using braid + 
foil is very flexible and fairly rugged.  That does make it 
handy/convenient if not ideal for interconnect cables and also feed 
lines to my 40 meter slopers.  It also handles the legal power limit if 
the SWR is not high. As I operate the entire 160 and 75 meter bands the 
very high SWR encountered at the band edges pretty much eliminates the 
use of RG-8X

Although you didn't list it, LMR-600 can be found at pretty reasonable 
prices and not a lot more than the premium Belden cables of 15 to 20 
years ago. I paid $1.29 a couple years ago and I believe it is presently 
on the order of $1.50 to $1.60 per foot.  Typically it is thought of as 
one of those coax cables for UHF, but for price Vs IR loss I find it 
ideal for 160 and 75 as well. I use it for all runs to and up the tower. 
I find that important as my overall runs from the rig to antennas are 
roughly 228', but that includes the rotator loops and feed lines (and a 
lot of connectors) from remote coax switches to the slopers where I now 
use either LMR-400 or Davis BuryFlex(TM)

There are two drawbacks to using LMR-600 and the larger coax cables for 
the average ham. One is its size/weight and the other is the price of 
connectors.  UHF can be found, but for the most part they are 
*extremely* expensive. (on the order of $80 to over $100 each), BUT 
Davis found some for me quite reasonable.  They are "off brand", but 
they were sent to me on an approval basis.  Testing showed them to be 
viable unlike many of the off brand hamfest connectors.  N type 
connectors are, or were on my last purchase, on the order of $13 (give 
or take a tad)  I've found N type connectors do not tolerate the high 
SWR at the band edges of 160 and 75 when running the legal limit. I've 
also had them blown out from nearby lightning strikes. A double female 
connector cost me a lot of time and money as I tore every thing in the 
system apart before it dawned on me that the connector might be bad.  It 
still looks brand new, but an ohmmeter shows a dead short from center 
conductor to outside.  I keep it for illustration, but it has a band of 
red around the middle just so it won't get reused.

They may be a bit more money, but I think I'd go with 7-16 DIN 
connectors on the LMR-600 in the future....after I use up the stock of 
connectors I have on hand.

73

Roger (K8RI)
> At the end of the section on coax, there's a table of various cable
> types, with costs listed. It's intended to help you answer exactly this
> question.
>
> 73, Jim K9YC
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