[TowerTalk] 80 Meter yagi question

[Jim Thomson]

Date: Wed, 11 Oct 2017 17:10:04 -0700
From: "Richard (Rick) Karlquist" <richard at karlquist.com>
To: Jim Thomson <jim.thom at telus.net>, towertalk at contesting.com
Subject: Re: [TowerTalk] 80 Meter yagi question


On 10/11/2017 10:11 AM, Jim Thomson wrote:

> ##  Aluminum only  conducts  60% as good as copper.  .25 inch OD copper  =  .417 inch OD  aluminum.

> Jim   VE7RF
> 

<That is only true at frequencies below which skin effect is
<significant.  For RF frequencies, you need to take the
<square root of 60% which comes out to 77%.
<1/4 inch copper = 5/16 inch aluminum.

<Rick N6RK

##  are u sure about that ?   K6STI coil software doesnt depict that effect.  Nor does other software.... unless they
are both wrong.   I also used Termans equations..and did it by hand.   They all concur.   This was calculating ESR. 
That was done at 7.1 mhz.   I was only interested in 40M  at the time...so did not  try higher freqs.    I did try 80m,
and got the same results.   Al  vs  CU.   In this case, the AL tubing had to be aprx .642  OD  to equal the ESR of 
.375  CU tubing.   Closest thing to that is .625..which is readily available. 

##  I also tried removing the cu tubing tank coil in one of my amps..and replacing it with an AL tubing coil... and both are the 
same  .25 OD, and aprx the same wall thickness.   Uh was identical, as was the turns, spacing between turns,  and also OD
and ID of new AL tank coil.   Did not have to touch the tune and load either, dead on.   Aluminum  coil ran hot.....  go figure.  
Those tank coil tests were all done on 15M. 

##  alumoweld wire..and also copper clad or copper bonded steel is another issue.... but I have not done any exhaustive tests on those..yet.
I did notice that stuff like  RG-303  with its silver plating, which is over the copper bonding..which is over the steel core..... on software, 
appears to flat line about 9.2 mhz and below... power wise.   My guess is, the  skin depth is deep enough such that the current is now into the steel core. 
RG-400  doesnt have that issue,  nor does RG-142.   RG-304 also has a steel core..same issues. 

##  But what the heck do I know,.. I just experiment.  Besides,  copper tuning is cheap at any home depot or supply house.   Then I silver plate it
with the cool-amp goop...just so it looks nice.  Bare copper drives me nuts, I lose sleep over it, looks un finished to me. 

Later.... Jim   VE7RF