[TowerTalk] Antenna Distance From Shack
Robert Chudek - KØRC
k0rc at citlink.net
Fri Aug 18 17:19:43 EDT 2006
Regarding your measurement at 28 MHz idea...
Last winter I wanted a 1/2 wavelength feedline for 1830 KHz. I used my AEA HF VIA Analyst to tune the coax. The AEA has a computer interface and allows charting of measurements so I could accurately monitor the changes as I snipped the cable shorter. This worked great and I nailed it right on my target frequency.
I created an Excel spreadsheet to track the changes as I snipped the coax. You can get a copy here: http://tinyurl.com/ob78y Grab the file named "160m Coax stub tuning" On 1.8 MHz the 1.01:1 VSWR points are plus/minus 30 KHz (60 KHz total) so the null is pretty broad on 160m.
The spreadsheet will show you the four snips I made and the resulting values after each change. BTW, just for reference, removing 1 foot of this particular coax (RG/8x no-name brand) moved the resonant point up 7 KHz. YMMV.
I was also under the assumption the feedline would work on even multiples of the fundamental frequency. What the meter showed me was the null points skewed as the frequency increased. It shifted up several hundred KHz each time the frequency doubled. You can imagine in the 10m band it was way off. Well, actually, you can see precisely how far off it was in the spreadsheet.
No one ventured to explain the skewing phenomenon I was seeing. What I learned from this exercise was to tune the lines at the intended frequency of use.
Regarding the complex impedance measurements and figuring coax loss... Yup, I was asleep at the stick again (talking, not doing)... Tom W8JI and your MFJ manual are right, of course. I was thinking of my coax tuning setup where I used the 50 ohm terminator, but not on the far end! The 50 ohm terminator goes on the analyzer end with a T-connector to connect the coax under test, the terminator, and the meter. The far end remains open.
73 de Bob - K0RC in MN
----- Original Message -----
From: "Pete Smith" <n4zr at contesting.com>
To: "Robert Chudek - KØRC" <k0rc at pclink.com>; <towertalk at contesting.com>
Sent: Friday, August 18, 2006 2:06 PM
Subject: Re: [TowerTalk] Antenna Distance From Shack
You're right, Bob, it was a typo.
The idea was that I wanted to have a half-wavelength (or close to it) at the lowest frequency of operation, meaning 160 meters. But I also wanted to measure as accurately as possible, which meant measuring at 28 MHz. I used an RG-11 pigtail so that I could get the length within an inch or so of being x half-waves at 28 MHz.
As for the loss measurements, W8JI told me that with a composite feedline like this, I could not get a usable loss number. The 259B manual says "be sure the distant end... is not terminated in any resistance" and also says that non-50-ohm systems cannot be measured.
73, Pete N4ZR
At 01:23 PM 8/18/2006, Robert Chudek - KØRC wrote:
>Can you elaborate on the "1750 Hz (measured at 28 MHz) of 3/4" CATV" you mentioned? First, I assume you meant 1750 KHz, but aside from that, I'm not following what you mean.
>Also, measuring loss at the shack with the MFJ-259B... Unless you install a non-resonant 50 ohm terminator at the opposite end, your readings in the shack will continue to 'vary widely' (although maybe not as "widely"). If you installed a 50 ohm load on one port of a remote antenna switch that would accomplish the task, but I'm thinking this is not what you had in mind regarding measuring loss.
>73 de Bob - KØRC in MN
>Date: Fri, 18 Aug 2006 08:13:14 -0400
>From: Pete Smith <n4zr at contesting.com>
>Subject: Re: [TowerTalk] Antenna Distance From Shack
>To: TOWERTALK at contesting.com
>Message-ID: <220.127.116.11.2.20060818080204.05175928 at mail.adelphia.net>
>Content-Type: text/plain; charset="us-ascii"
>Frank and Bill make good points. For the last 10 years, I have been using 3/4" CATV for most of the 190-foot run from my house to my antennas. To be specific, I use RG-213 up the tower and across the roof of my house to my 2nd-floor shack, but use multiples of a half-wave at 1750 Hz (measured at 28 MHz) of 3/4" CATV in between.
>-- some stuff snipped here --
>I want to be able to monitor loss from the shack, something that my composite feedline doesn't seem to let me do (the indicated loss on my MFJ-259B varies widely with frequency).
>-- trailing text snipped here --
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