[TowerTalk] RG142 Measurements for chokes

[Grant Saviers]

A concern was posted about how well RG142 silver plated copper clad 
steel center conductors would perform at frequencies below 10MHz.  A 
good question, especially since I use 142 in chokes for all bands.

I have a spool of RG142 by Thermax, a bit old, but bought (really cheap) 
as real Mil surplus and marked MIL17/158-00001.  The tag says the spool 
started as 933 ft.  So I set out to measure the resistances and loss.  A 
4 wire 6 1/2 digit DVM was used to measure ohms and a VNWA 3e to measure 
loss, 1 to 30MHz.

The conductor resistance is 11.602 ohms and the shield resistance is 
1.430 ohms.  If I use the Belden spec of 64 ohms/km for the center 
conductor then my spool has about 600 ft remaining.

Using my VNWA the loss in db is

MHz  db
1      2.18
1.9   2.99
5.0    5.28
10    7.53
30    13.14

The trace is flat db vs Frequency beyond 10MHz and shows an increasing 
curve below 10MHz due to the higher conductor resistance.  It appears 
that the copper cladding plus silver plating is thick enough to work at 
frequencies > 1MHz.

If my calculated ft remaining is correct at 600 feet, then measured loss 
per 100ft is currently 2.2 db at 30Mhz, 1.25 db at 10MHz and 0.5 db at 
1.9Mhz.  The Pasternack spec is 1.20 db/100' at 10 MHz, so all looks ok 
spec vs measured for a 30 year old spool of coax. With about 5' of RG142 
in a tribander choke, the loss is < 0.09db and for 10' in a 160m choke, 
< 0.08db.

So there is no reason to be concerned about the slightly higher loss of 
RG142 below 10MHz, it is insignificant in choke.

The Pasternack spec is at 
http://www.datasheetarchive.com/dlmain/SFDatasheet-2/sf-00045325.pdf 
(btw elsewhere the isolation is specified >90db, due to the double shield)

email me off list if you want the VNWA plot.

Grant KZ1W