[TowerTalk] Change in Frequency As Antenna Height Rises

[Grant Saviers]

A little sloppy, I should have said "transforms to some other SWR on an 
SWR bridge".

We agree there are a couple of much better measurement means.  Your 
"lacking that" example still requires a measuring device that can do TDR 
and complex impedance measurements which a classic "SWR Bridge" can't do.

It still seems a bit bizarre to me to be tuning individual elements of a 
beam near the ground with an SWR bridge. Also, several classic 
"great/proven" beam designs have been redesigned with good modeling for 
much better performance out of the same pile of aluminum.  And some well 
known brands showed zero gain in the K7LXC et al yagi tests raising big 
questions about "manufacturer's data.".  It's gonna be hard to convince 
me that there is a better way than good modeling to design a beam.

Grant KZ1W

On 5/31/2016 14:40 PM, Jeff DePolo wrote:
>> Perhaps this debate is really a measurement problem ---
>> Perhaps this debate is really a measurement problem ---
> Part of the problem is that some people consider "resonance" and "matched to
> the transmission line" (i.e. 1:1 VSWR) as being one in the same when they
> are talking about antennas, when they most decidedly are not.
>
>> What is needed is an analyzer that can be calibrated with the
>> measurement plane at the far end of the coax.  A VNWA,
>> AIM4170, SARK110,
>> etc. can be calibrated with S/O/L on the end of the cable so the
>> measurement plane is at the antenna.  Then when jX = 0 you have
>> resonance.  However, even then real R might not be 50 ohms, not 1:1
>> SWR.  Otherwise some complex Z probably transforms to 1:1 SWR
>> on an SWR
>> bridge at some other frequency than Fres depending on the
>> length of coax.
>>
>> Grant KZ1W
> Maybe I'm not understanding what you were saying, but there's no way a 50
> ohm cable can transform a load Z that's not 50+j0 to yield a 1:1 VSWR (ref
> 50 ohms).  Please clarify.
>
> I agree that the most reliable way of making feedpoint Z measurements is to
> move the calibration plane to the end of the transmission line.  But lacking
> that, you should be able to get fairly accurate in situ feedpoint Z data by
> starting with an accurate measurement of electrical length of the line
> (using TDR) and then rotating the Z measured at the source end of the line
> accordingly.  Of course you have to factor in line loss as well, which is
> best determined via a return loss measurement at the frequency of interest
> with the line open/shorted (take the average of RL with the line open and
> shorted, and then divide by 2 to get loss), but lacking that, manufacturer's
> data is the next best thing.
>
> 				--- Jeff WN3A
>
>
>
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