Date: Wed, 11 Oct 2017 17:10:04 -0700
From: "Richard (Rick) Karlquist" <richard@karlquist.com>
To: Jim Thomson <jim.thom@telus.net>, towertalk@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
### I checked my notes on this.....plus yours you posted. on that url. Here
is where I think the issue is.
-AL only conducts 60% as good as Cu...at any freq.
-14 gauge solid Cu = 12 gauge Al. (standard for house wiring etc, 60 hz or
dc)
- 14 gauge = .0641 diameter
- 12 gauge = .0808 diameter
- .0808 / .0641 = 1.26 Increase the diam by 1.26 and now the cross
sectional surface area has increased by.... 1.26 X 1.26 = 1.59 or almost
60%.
- Go to page 132 of ur url....under skin effect.
http://www.americanradiohistory.com/Archive-Handbooks-Miscellaneous/Federal-Reference-Data-Radio-Engineers-4th-1956.pdf
By comparison, a thick aluminum sheet has a resistance 1.28 times that of
copper.
- notice the correlation between their 1.28 and the 1.26 from the 14 / 12
gauge house wiring example. But in both cases, solid material is used.
- Cuz of skin effect at HF + VHF.... Copper or aluminum tubing will
suffice. Typ copper tubing is aprx .035 thick.
Look in at the END of a piece of thin walled cu tubing.....and you will notice
that the total surface area of the
end of tubing is miniscule at best. We still have to increase the miniscule
surface area of the EDGE of the
tubing.... if aluminum tubing is instead used. And to increase the surface
edge area by 60%..we now have to
increase the diam by 60%. If the skin depth is only say .05 and the Al
/cu tubing is .035 thick, the excess is wasted,and IMO, does not count
as...useable surface area.
So you still gotta increase the diam of any AL tubing by 60%. A 60% increase
in diam will obviously result in a 60% increase in circumference.
## .25 OD copper x 1.6 = .4 OD AL.
.375 OD Cu x 1.6 = .6 OD AL.
.25 OD AL = .156 CU = 5/32 inch Cu. Now you can see why .25 AL
tubing will run blazing hot when used in an amp that previously used .25 Cu
tubing. .375 OD AL tubing would be required for the job. Al cant be
soldered, if soldering is required. Tubing is usually flattened at both
ends..and a hole punched through, then machine screws, etc used.
## per W8JI and his eng buddies, RF only flows on the outside of tubing,
and not the inside. That has to be factored in. Paragraph 1 on page 128 of
your above url alludes to that.
Strap, on the other hand, conducts RF on both sides. Strap is mentioned in VO
Stokes book.... Radio Transmitters + power amplifiers, published in the UK.
Strap has to be the same total circumference as the tubing ..to
handle the same RF current. .25 OD cu tubing is .785 inch in circumference.
If cu strap used, and is .035 thick, it only has to be .3575 wide. IE: .25
cu tubing = .3575 cu strap. Strap has some other advantages, like if flat
wound strap coils are employed. There is no C between turns of strap coils,
hence adjacent turns can be very close together.. but strap coils are typ used
on 12-10-6m amp tank coils. But Im slightly OT here.
## The software from k6sti and others depicts the same 60% required
increase in OD, when swapping from AL to CU.
Jim VE7RF
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|