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Author Topic: Analyze Vertical Antenna at Base or in Shack?  (Read 2904 times)
K0IZ
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Posts: 737




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« Reply #15 on: March 29, 2013, 03:26:46 PM »

Dale, good points.  His ugly balun is wound around a 5gal pail.  That's pretty big diameter for upper freqs, probably 6 or 8 inches would be better.  Using analyzer at antenna, I would be inclined to use a short, 1ft or so, jumper, with a number of ferrites on it
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N4NYY
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Posts: 4745




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« Reply #16 on: March 29, 2013, 03:40:53 PM »

Dale, good points.  His ugly balun is wound around a 5gal pail.  That's pretty big diameter for upper freqs, probably 6 or 8 inches would be better.  Using analyzer at antenna, I would be inclined to use a short, 1ft or so, jumper, with a number of ferrites on it

A 4-5" PVC coupler is ideal
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K0ZN
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Posts: 1543




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« Reply #17 on: March 29, 2013, 04:00:15 PM »

You need to connect the analyzer right at the base. Any coax between the analyzer and the antenna feedpoint will throw off readings, especially if there is a little mismatch.
You *probably* could connect something less than a foot and have minimal impact, but it is best with the shortest connection possible.

One other option is to use an ELECTRICAL half wave length of coax on each band. (Kinda expensive just for test fixtures!)  An ELECTRICAL half wave of transmission line exactly repeats the impedance at the other end, thus you "see" what is going on at the base. Personally, I would just connect the meter at the base.

73,  K0ZN
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WB6BYU
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Posts: 13120




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« Reply #18 on: March 29, 2013, 04:30:37 PM »

Or, if you know the length of your coax, you can calculate the actual
impedance at the feedpoint from the measured value at the shack end
of the coax using VK1OD's transmission line calculator:

http://vk1od.net/calc/tl/tllc.php

No need for a cable that is 1/2 wavelength long.


But if you are looking for the frequency of the minimum of the SWR curve,
that will remain constant regardless of mismatch when you add coax, as
long as the coax is the same impedance as the analyzer.  While the
impedance varies along the feedline, the SWR doesn't, except
for a reduction due to cable losses.

In most cases, adjusting an antenna for minimum SWR at the shack end
is the most effective (and less error-prone) method.
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KB1GMX
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Posts: 745




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« Reply #19 on: March 29, 2013, 07:40:19 PM »

My $0.02 from building and testing lot of antennas. 

When you used the 6ft length that puts you right next to the antenna holding
the end of the coax via the antenna analyser.  That's nothing like how the
antenna is going to be used.  You become part of the environment and
likely made the antenna tune lower then if you were much further away.

The ugly balun is way to large around.  a 5-6" piece of something is about right
with at least 10-20 turns unless your using something unusually stiff.

Note unless the balun has _very good _choking action the shield of the coax
is part of the radial farm.  A second one close to the entrance is advised.

Usually when a vertical shifts like that it suggests the "ground" isn't.  Add
more radials.

Tune it for the best match at the rig, it s the radio/amp your trying to
make happy.

100Ft of decent coax like quality RG213 isn't enough loss to make a big deal
about when measuring SWR unless the stuff is damaged or bad connectors.
This is HF, if you have more than 1db of loss in the system something is broke.

If you want an RF short (pl259) use a connector and a barrel (DBK female to
connect it) and the connector has a #12 wire from the pin to the shell
(very short, direct!).  A short like that is handy for testing stubs and DC
testing for open cable.  A small dummy load (connector with a short leaded
quality 50 ohm resistor) is handy too.  A clip lead is another word for a crappy
electrically short antenna.

Making the cable a half wavelength is pointless for an antenna that covers at
80 through 10M including 30M and 15M and is more than 50FT away.


Allison


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W5DXP
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Posts: 3551


WWW

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« Reply #20 on: March 30, 2013, 09:05:29 AM »

a 5-6" piece of something is about right with at least 10-20 turns unless your using something unusually stiff.

Twenty turns using a six inch piece of coax??? What diameter coax does one use?Smiley
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K8AXW
Member

Posts: 3722




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« Reply #21 on: March 30, 2013, 09:28:05 AM »

I always tune my antennas at the radio.  I never change the length of my leadin which goes through a LPF, SWR meter and scope before it gets to the radio.

When I tune the antenna for a low SWR at the radio, the radio seems to be happy.  Other than a vertical, I've never been able to tune with the analyzer or SWR meter at the antenna.

FWIW
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K0ZN
Member

Posts: 1543




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« Reply #22 on: March 30, 2013, 09:58:40 PM »


OK.... to respond to some of the foregoing comments:

Re: the half wave length of coax. I just said that to mention it *could* be done since a half electrical wave does repeat the impedance.
I did NOT say it was the best, most economical, simplest or most effective way.....  it is just *one* way to make the measurement.
I DID say you would need a length for each band and that was expensive (DUH!).

Since the rig sees the Z at the shack end of the coax, obviously, measuring in the shack allows one to see what the rig "sees" for Z.

73,  K0ZN
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