AND...they re-use the sawdust for particle board and sell it to someone
else!
73,
Steve
N6SJ
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of Richard
(Rick) Karlquist
Sent: Saturday, September 26, 2020 8:46 AM
To: Artek Manuals <Manuals@ArtekManuals.com>; towertalk@contesting.com
Subject: Re: [TowerTalk] Real "50 ohm" cable ?
1. There are various ways to measure characteristic impedance, some are
better than others. You didn't say which method you used. I have the most
confidence in terminating the coax with an accurate 50 ohm load and
measuring the input impedance ripple over a band of frequencies. I learned
this from W8JI, who is nearly always correct. I have not had good luck
measuring inductance of a shorted line and capacitance of an open line at a
spot frequency.
2. Many RG-XX specs don't even claim to be exactly 50 ohm.
3. The RG specs don't cover foam coax, but you will see "Foam RG-8U" etc,
whatever that means. It is a given that foam coax is never precision coax.
4. Foam is difficult to manufacture with precision, because the amount of
air is hard to control. OTOH, foam has significantly less loss, so in
general, you want to use it.
5. Specs on "velocity factor" should be interpreted as "minimum",
especially as stated above where foam is used. Even then, always check VF
if it is important to you. I bought some coax for phasing lines from a
vendor with a good reputation. However, its VF was less than the spec, and
I couldn't shorten it to compensate because then the length wouldn't reach
from point A to point B. Admittedly, I represented the proverbial corner
case, but the vendor made good on the purchase.
6. To further complicated the issue: Zo is actually complex and
"dispersive" meaning it varies over frequency. Steve Stearns, K6OIK, has
written some excellent papers on this topic that are up on the web. He has
shown how you can get erroneous results if you don't do calculations taking
into account the complex value of Zo. It's not sufficient to just model it
as a real impedance cascaded with an ideal resistive attenuator. This
actually causes more trouble in practice than small deviations in the
magnitude.
Rick N6RK
PS: 2x4 lumber consumes 8 square inches out of the original log.
You pay for the sawdust!
On 9/26/2020 6:53 AM, Artek Manuals wrote:
> I recently purchased a nice VNA ( Array Solutions AIM4300 ...good news
> /bad news ..contact me off list) . Like a kid with a new chemistry set
> at Xmas I set about measuring everything in site with my new "toy".
> Much to my naive surprise I found that much of the "50 ohm" coax I had
> (DX engineering RG213 and RG8X), actually measured 51.8 ohms .
> Although I did have a 75 ft piece of brand-X Hamfest special that measured
49.7 ohms!
>
> Now I know that in practice there is not enough difference between 50
> ohms and 52 ohms in actual use especially at 50MHz and below.
> Occasionally though it matters ! Besides the truth in advertising
> thing ( 2x4 lumber has always galled me) Does anyone know who makes
> legal limit 50 ohm coax that is actually 50 ohms and not 52 ohms?
>
> Dave
> NR1DX
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