[bars] Impedance of yagi

Sat Sep 10 12:31:27 EDT 2022

Hiya Geoff,

Apparently I qualify as being 'of a certain age'. ;-)  I def. remember 
'rabbit ears' and the gyrations folks went through to pick up those 2 or 
3 channels.

I did those separate measurements to validate that the DE was even close 
to right before I proceeded with the rest of the build. Based on the 
various replies I've read the director spacing is likely a bit off as 
well as the fact that I can not feed it directly with 50m coax and 
expect a 1:1 match.

I'm going to try it with, and without, the director this wkend to see if 
it makes a significant difference.  I'll leave the match to the rig's 
built in ATU for now.

I'll put the nanoVNA back on it later and check the match across the 
band.  I was focusing on the FT8 section when doing initial tests.

So far, without the director the performance is less than grand on FT8.  
I've got it about 10ft above ground on a mast strapped to my deck 
railing. ;-)

73,

Tim - KC1RET

On 9/10/2022 10:10, Geoffrey Feldman wrote:
>
> People of a certain age might remember Rabbit ears.   Folks would do 
> all kinds of weird things to get the station we wanted. Lengthening, 
> shortening, adding bits of aluminum foil. Almost nobody understood 
> antenna resonance but they had some idea of changing the antenna made 
> it better or worse.   The other humorous factor is that stepping away 
> from the antenna, might make it worse again.  Often friends and 
> relatives were forced to stand next to the antenna, unable to see the 
> screen so the rest of us could see whatever.
>
> Impedance depends on frequency.  That depends on what else is nearby 
> the antenna.  The director and reflector, as is everything around, 
> part of the SWR.   Measuring the driven element alone or measuring it 
> with the reflector, then adding the director is a waste of time in a 
> Yagi.  Put the antenna together as designed for the band you want, 
> then measure. Also make your measurements with the antenna at the 
> height and in the environment you anticipate.
>
> It would be interesting to know if the 2.5:1 is true for all 6m 
> frequencies.  The impedance will not be the same at all frequencies 
> either.  That’s also not a horrible SWR, not great but not horrible.  
> A tuner can compensate for the rest.  Isn’t there a function in the 
> nano to chart impedance / SWR across an entire band?  If someone has a 
> rig-expert that can do it for sure.
>
> -73-
>
> W1GCF
>
> Geoff
>
> *From:*bars [mailto:bars-bounces at w1hh.org] *On Behalf Of *Tim KC1RET
> *Sent:* Saturday, September 10, 2022 9:54 AM
> *To:* bars at w1hh.org
> *Subject:* [bars] Impedance of yagi
>
> So I'm trying to throw together a 3el 6m yagi for the VHF contest this 
> weekend. I built one based on a design for a wire + PVC beam I found 
> on da Google (http://ehpes.com/n6mw/WireYagi2.pdf). Mine has a bit 
> less duck tape. ;-)
>
>
> I tested only the driven element with my nanoVNA, spot on 1.1 at my 
> target of 50.2 (FT8). I added the reflector - still 1.1. (All 
> measurements taken at about 10ft. elevation). I added the director - 
> the lowest SWR is now 2.5:1 at the same freq. (the SWR low point did 
> not change).
>
>
> The measured impedance dropped from 50.2 to around 30 after adding the 
> director. I took the wire off the director element support (aka PVC 
> pipe) and the SWR went back to 1.1:1.  I did check that the elements 
> were parallel (as best as can be accomplished with PVC tubing and bits 
> of cutting board).
>
>
> What variables in the director (length, spacing from DE, etc.) would 
> cause this?
>
> 73
>
> Tim - KC1RET
>
> -- 
>
> Courageously sent from a non-Apple device that /includes/ a headphone 
> jack.
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>       Tim Carter - KC1RET
>
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> +1.978.710.0133 <tel:1.978.710.0133>
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-- 

Courageously sent from a non-Apple device that /includes/ a headphone jack.

------------------------------------------------------------------------
<https://kc1ret.com/img/kc1ret-qsl-card-front-v3.png> 	

      Tim Carter - KC1RET

	+1.978.710.0133 <tel:1.978.710.0133>

	kc1ret.radio at gmail.com

	https://kc1ret.com

"Computers are incredibly fast, accurate and stupid; humans are 
incredibly slow, inaccurate and brilliant; together they are powerful 
beyond imagination." -- /Albert Einstein/

Warning: Spelling errors in this message are the product of a poor 
school system. Pay teachures more than athletes.
IMPORTANT: This email is intended for the use of the individual 
addressee(s) named above and may contain information that is 
confidential privileged or unsuitable for overly sensitive persons with 
low self-esteem, no sense of humor or irrational religious beliefs. If 
you are not the intended recipient, any dissemination, distribution or 
copying of this email is not authorized (either explicitly or 
implicitly) and constitutes an irritating social fauxpas. No animals 
were harmed in the transmission of this email, although the mutt next 
door is living on borrowed time, let me tell you.
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