The AM Forum

Hello Again,
I have recently refurbished my A3S Yagi and very impressed with the results. BUT 5 yrs in the air and the antenna looked like it was rode hard and beaten. Loose elements, plastic caps over the traps shredded, WATER in the boom!!, and some measurements out of spec.(my fault) ( A very bad time in my life!!!)
Sweeping the various bands I see respectable SWR, but the MFJ reveals pretty high numbers of reactance. And The Rs can swing from 25 ohms to 80 ohms on some bands, even though the "SWR" will be within the 2:1 points.
I'm looking at the 2:1 points for the usable parts of the bands.
Will a Yagi display these weird numbers?

Thanks

Fred

Fred,

It depends on where the reflector and director elements are tuned.   Tight tuning (near the resonant operating freq of the driven element) produces better f-b and gain, but results in a narrower swr and f-b/gain curve.   

With your Yagi, the tuning of the trap freqs and especially the lengths of the tubings will dictate these curves.

Trap antenna manufacturers usually publish the swr and f-b/gain curves for various bands - and sometimes have recommended settings for the CW or phone only portions.   Did you write down your settings and can now compare them to the MFJ-259?  This will tell you if you have a problem like contaminated traps or tubing that has slipped out of place.  Were the curves better when the Yagi was newer in the past?

Yes, a tightly tuned Yagi will show a good swr curve on a narrow band of freqs only, like maybe only 50kc and then soar outside of it.  In contrast, if the reflector and directors are tuned far away from the center driven element operating freq, then the swr curve will be much broader.  Think of a dipole alone. The curve is very broad. As you add elements and tune them closer to the dipole's res freq, the interaction become larger and the swr curve becomes steeper.

T

Multi band trap antennas are a compromise to start with. The published VSWR curves almost never say they start at any pure resistive number and the curves you see and read on a standard SWR meter (including any ricebox radio) are a combination of reactive and resistive numbers.

The MFJ simply shows you what the advertised specs really mean.

Carl
KM1H

To be fair, this is true of any antenna. Reactance is only zero at exactly resonance. And all antennas are a compromise of one sort or another.

Donno your location but if your antenna is picking up some kind of signal from somewhere else, that can screw with the readout on the MFJ. 

To be fair you don't need to preface that statement with "to be fair."

LOL. I didn't even NEED to post.

I felt trap antennas were being unfairly bashed. It's my mission in life to protect the down trodden trap antennas. And, of course, the children too.

Trap antennas as implemented in real life could use some protection and support. Thank you Steve.

Trapped aerials are so misunderstood.

I was just noticing the numbers changing as I swept through the 2:1 points and saw athe varying amounts of reactance. When first installed I just saw the 2:1 SWR's and was happy. As I get older I see more things. A lot of magic taking place.

Interesting replies.

Fred

It's a trap to think all trap antennas are fair.  Trap antennas represent both the good and evil in antennas. I've known trap antennas that would give you the skin off their traps. While others won't give you the trap of day.

T

SWR's? Fred is that you or an impostor from outer space?

I like the way hot traps drip plastic like so many 70s beer can plastic 6 holer retainers lit afire by carefree dropouts. Nothing compares to flaming plastic moving through time and space.

[
Maybe Fred took readings on several bands, thus "standing wave ratios" ?  ... ;)

I live near Grover's Mills. Can't be too careful.

I hear guys on the air often talking of their SWR (SwerZ).

Yeah, I hear that often too. It's a carry over from the good buddy band.

I wonder if "SWR's" shud really be "Standing Waves Ratio" ... just like the plural  "Attorneys General"  ??  .... ;D

Steve said:

I SWerZ quite often!!!  ;)

When I worked at MaCom, the expression was Vizwaa

(VSWR - remember in New England a trailing R is replaced by the long a in speech). 

Did ya get the Vizwaa plots on that S-band antenna Ed?
A-yuh, an it failed at 1.97 to 1 too, just like that.


Sounded much cooler that way, but please no French Accents!!!   ;D

You were OT if you know how to use a slotted line too. ;D

• To be fair, this is true of any antenna. Reactance is only zero at exactly resonance. And all antennas are a compromise of one sort or another.

True but to be fair and not blow off on a tangent I was refering to the habit of some trap antennas to give readings that are not as expected compared with a monoband dipole or yagi.

Carl
KM1H

Whether it's unexpected or not depends on who is doing the measuring.   ;)

I though I would throw a curve ball abou the SWR's or SWERZ. Stuck with the off shor radio and the foldback circuits. I know we don't worry about losses at the MF and the lower HF, but wouldn't high vizzwazz cause trap failure? Or heating?

Fred

With a Cushcraft all bets are off and its only going to get much worse. :o

Carl
KM1H

How about coaxial traps?  I hear mixed reviews but they should be durable enough for the full legal limit if made of RG-213 or equiv solid dielectric cable.
Relatively easy and cheap to build too.