[bars] Impedance of yagi

Sat Sep 10 10:10:10 EDT 2022

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

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