[Amps] brass or BNP hardware

[Jim Thomson]

Date: Fri, 19 Nov 2010 13:36:22 -0700
From: "John Lyles" <jtml at 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