[TowerTalk] Topband: Measuring resonance of a yagi element

[Wes]

Rick is correct.

Any easy way to see this is to use Dan, AC6LA's, TLDetails program.

Wes  N7WS


On 4/26/2020 1:38 PM, VE6WZ_Steve wrote:
> Rick,
> We are talking about sweeping the feedline and looking at the REAL R sweep vs frequency….NOT impedance Z=R+jX.
> We are looking for relative dip in R, not absolute values.  We are looking to see at what frequency this dip occurs.
>
> If I put a 25 ohm resistor and then a 50 ohm resistor at the end of a 55 degree feedline, are you saying the real R (not Z…no X) will read 50 ohms in both cases?  I think my multimeter would say something different at DC.
>
> If this is true, then you are correct, and the dip in the real resistance seen at the driver at the parsitic frequency would be muted….the sweep would be flatlined at 50 ohms across the frequency of interest.
>
> de steve ve6wz
>
>
>
>> On Apr 26, 2020, at 12:35 PM, Richard (Rick) Karlquist <richard at karlquist.com> wrote:
>>
>>
>>
>> On 4/23/2020 7:40 PM, VE6WZ_Steve wrote:
>>> Tom,
>>> Yes.
>>> Just sweeping from the end of the coax in the shack should show the same dip of R at the correct frequency.
>>> (The absolute R value (in Ohms), and X value will not be correct because of the feedline transformation, but the frequency of the R dip is ONLY a function of the reflector tuning.)  Try it and see.
>> I don't see how this would be true for an arbitrary length
>> of coax, especially if it is an odd multiple of a quarter
>> wave.  For example, if the load on the far end changes
>> from 50 ohms to 25 ohms, the analyzer in the shack will
>> see resistance INCREASE from 50 to 100 ohms.  A line with
>> electrical length of 55 degrees will have a resistance
>> at the shack of 50 ohms whether terminated in 50 ohms
>> or 25 ohms.
>>
>> 73
>> Rick N6RK
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