Date: Fri, 19 Nov 2010 13:36:22 -0700
From: "John Lyles" <jtml@losalamos.com>
Subject: [Amps] brass or BNP hardware
In RF power amplifiers, HF to VHF range, I have used brass or nonmagnetic
stainless steel hardware for years, after having been "Burned" by steel screws
- pun intended. Several times in my career
I have seen bolts turned black and tarnished from RF heating this way. If RF
current can flow in the steel, then it can make eddy currrent losses and heat
up. However, if it is not carrying current, not a problem.
The reason I try to stick with brass or SS is that later on, someone is working
on the product, cannot remember why this special screw is only in this hole and
that. So by making it all one type, when inside the RF compartment, you
make service, repair and spare parts inventory simpler. These simple policies
are carried on in my mind when starting new amplifiers.
73
John
K5PRO
## In Bings infamous RF software, one of the features is the skin depth
calculator. It also has a table that list every kind of metal you can think
of. The worst of the bunch was SS. Per his
list, rusted bailing wire would be better than SS. He didn't say whether it
was magnetic, or non magnetic SS.... or if 304 SS or the cheaper
variety..which I think is 18 type.
SS appears to work fine, if used to clamp cu straps together, etc, where the
main RF current patch is not though the actual SS threads. IE: cu strap wrapped
around a tubing tank coil, brought back on itself, then
machine screwed, which pinchs the cu strap tightly, onto the tank coil.
Where I got into trbl was when double nutting SS machine screws and
placing strap between the two nuts. Then the RF path is now through the SS
threads
and stuff heats up..fast. [ I did this on a ceramic vac cap]
The SS that's locally available in common sizes like 4-40, 6-32, 8-32, 10-32
1/4-20 etc... will just barely stick to a magnet...and I mean barely...and
that's with a big magnet. If a small magnet is used, they won't stick at all.
N6BT once had a pix on his old F-12 website, where he tried using solid 8 ga
SSS wire for a 40m helical hair pin. The exact dead center of the coil
had turned jet black..over a +/- 1 " portion.
F-12 typ uses SS 10-24 hardware at feed point of their ants/yagis'. They
also double nut them... with the balun lugs between the 2 x nuts.
## I noticed on one of these sites that supplies ant materials for high power
AM SW broadcast and commercial users.... recommended sizes of hardware
for the feed points of various SW ants...
including curtain array's... rhombics, V beams, LP yagi's etc. It went from
1/4-20 and 1/4-28 up to real big stuff like 3/4"..and 1". It was a
table of power VS bolt size. Interesting enough... ALL the bolts
used consisted of phosphor bronze. None of it was SS / brass / or nickel
plated brass.....or anything else. I'm sure Mosley used phosphor bronze
screws for the feed-points of their yagis.
## so what's the story on Phosphor bronze? Is it really the preferred
material to use for RF ?? I have yet to try to see if I can silver plate
any of this stuff. Back in the 70's/ 80's and early 90's.. telco's
used copper bolts into tapped copper buss bars on a regular basis. [1/4"-20 +
5/16"] But I don't think they were solid copper. I think they were copper
plated material, but don't know what the base material
was. I didn't want to scrape any cu off... exposing the base metal, if they
were plated... as the CU bolts were scarce to start with, and we needed all of
em.
## Brass is plentiful at the local stores. I was surprised that they even
stocked huge 5/8" hex headed brass bolts, in various lengths. I have
never seen nickel plated brass... and never heard of it
till now. Phosphor bronze on the other hand, is readily available from a
variety of sources, in just abt any size. I have never had any problems
with silver plating CU. Dunno if it's even possible to silver plate say
phosphor bronze, SS, or brass/nickel plated brass. I don't like the way
SS interacts with Aluminum tubing or Aluminum plate. The AL always ends
up grossly discoloured.
later....... Jim VE7RF
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