First of all the 5 dB loss stated is at 26 MHz and not 3.9 MHz. There is a
difference. And with 1 S unit being 6 dB, I seriously doubt you'd notice
the difference. If you believe you do, just tweak the RF Gain up a bit.
My thoughts on 600 ohm vs. 450 ohm vs. 300 ohm feeding an almost resonant
dipole, the dipole impedance being 30 ft above ground at 3.9 MHz would be
about 30 ohms. For 600 ohm line the SWR on the line would be 20:1, for 450
ohm line the SWR would be 15:1 and for 300 ohm about 10:1. There are
several published charts that depict the added loss due to SWR for various
values of matched loss in lines.
For a line having a matched loss of 1 dB per 100 ft, the added loss with a
20:1 SWR would be an additional 4.5 dB, for the 450 ohm line having a loss
of 1 dB per 100 ft the added lose due to SWR would be 3.5 dB and for 300 ohm
line with a loss of 1 dB per 100 ft the added loss due to SWR would be 2.5
dB. I would seem then that 300 ohm line would be more efficient than 600
ohm line for the same parameters.
The general point being, open wire line will have a much lower loss
operating with a high SWR as compared to coaxial cable with the same
mismatch condition. Of course in the example used, the 30 ohm feed Z and 50
ohm cable would have a SWR of only 1.6:1. Typically at 4 MHz about any coax
one chooses will have less than 1 dB of loss per 100 ft when matched. And
at 30 MHz again coax loss for 100 ft of RG-58 would be some 3 dB when
matched. And RG-8 or RG-213 will have 0.3 dB of loss per 100 ft at 4 MHz
and 1 dB of loss per 100 ft at 30 MHz.
I realize that we'd like to eak out every dB we can, but in the end, it
makes little to no difference on HF.
73
Bob, K4TAX
----- Original Message -----
From: "Wade Staggs" <tvman1954@gmail.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Thursday, August 01, 2013 10:52 AM
Subject: Re: [TenTec] OT: Openwire/Window Line and Bad Wx
Hello Steve,
I don't doubt your instruments but, do believe that
we would notice a 5db drop in signal. Unless the books taught me wrong, a
5db loss would be Very Noticeable. About this, there is no need to ask
folks on the air with a signal report. A 5db loss would make a Major
difference to my signal meter. The station is already running with the
20db
attenuator on and the RF Gain reduced. Another 5db would be noticed. This
we are sure of. Steve, I am not saying that you are wrong, just saying
that
at the shack here. I would really notice the attenuation you are speaking
of. Also, here in the real world and not a Lab, I will be looking very
closely for that 5db of loss. You see Steve, with the 20db pad in and
reduced RF Gain already .... another 5db would really be noticed as the
Feed Line Dries or gets Wet. There must be another factor coming into play
with your testing. Either that or I have the best receiver in the world...
Thanks Steve for your report and we do look forward to seeing what the
other Guru's say about your findings.
73 from Wade/KJ4WS
On Thu, Aug 1, 2013 at 10:19 AM, Steve Hunt <steve@karinya.net> wrote:
Problem solved .... I think.
Just added some wetting agent - washing up liquid - to the water and
re-sprayed. Losses jumped to 5dB/100ft at 26MHz!
Steve G3TXQ
On 01/08/2013 14:58, Rick - DJ0IP / NJ0IP wrote:
Steve,
I hope you document the test on your web site.
It is extremely useful for people.
I'm afraid papers like Wes' scare people away from using commercial
window
line and most people are afraid to build their own operwire line, so in
the
end many people miss out on the benefits of using this type of feedline.
73
Rick, DJ0IP
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