[Antennas] Elementary Antenna Question

[Terry Conboy]

Perhaps there is some confusion between impedance and resistance in the 
readings of the MFJ-259.  For example, a load impedance of 35.4 +j 35.4 
ohms has an impedance magnitude of 50 ohms (with an angle of +45 
degrees), but since it's not resistive, the SWR would be 2.46:1.  Same 
for a capacitive load of 35.4 -j 35.4 ohms, except the angle of the 
complex impedance is -45 degrees.  There are an infinite number of other 
impedances with a 50 ohm magnitude, and all but one have an SWR greater 
than 1:1.  The ONLY impedance that will give an SWR of 1:1 is 50 ohms 
that is purely resistive with NO +/-j reactive component.  Period!  If 
your test equipment gives a different answer, it is functionally defective.

And don't forget that the impedance at the input to your feedline (with 
or without any 1/4 wl 75 ohm line) will be rotated around the Smith 
Chart by line length and impedance, so you have to back the impedance 
transformation of the line out to figure out what the antenna itself 
looks like.  The TLW program that comes with the ARRL Antenna Book is 
great for this.  But if you do see 50 ohms (purely resistive) at the 
input to a 50 ohm line, the load impedance will be darn close to 50 ohms 
resistive, especially after adjusting for line losses.  High line loss 
makes the input SWR look better and the input impedance become closer to 
50 ohms resistive, even if the load isn't quite a perfect match.

As others have pointed out, antenna efficiency is a function of losses 
(wire, feedline, ground system, loading L or C, etc.).  It really isn't 
related directly to SWR.  The usual counterexample is a good 50 ohm 
dummy load, which has SWR=1 but the efficiency as an antenna is a 
fraction of a percent, and only if it leaks a little RF into the ether.

Furthermore, some power amplifiers actually put out more power into a 
load somewhat different than 50 ohms.  Even though the amp may be 
designed for a 50 ohm load, the output transformers and lowpass filters 
aren't perfect, and a slightly reactive load a bit off of 50 ohms might 
allow the amp to deliver more power and/or have higher efficiency.  All 
this depends on the frequency, of course, and it's never documented.

Also, it isn't so much of a problem on 20m, but the MFJ-259 has a 
wideband detector, so strong signals from local AMBC or other 
transmitters can make for some pretty strange readings, although this 
doesn't appear to the issue here.  But on 160m and 80m, it is often a 
real pain.

73, Terry N6RY

On 2009-10-01 5:59 PM, WX5L wrote:
> Hello
>
> I was thinking about antenna efficiency and was sweeping my quad with my
> MFJ-259 and was curious with the findings. 
>
> Basically why does the antenna resistance(50 ohms) does not correspond with
> the SWR dip?
>
> For example in my case........I have a 50 ohm reading with the MFJ at 14.025
> with a SWR of 1.5 to 1 but the SWR dips to 1.2 to 1 at 38 ohms and at
> 14.325.
>
> I would think the antenna would be most efficient at a 50 ohms reading
> rather than 38 ohms even with a lower swr. 
>
> I am not using any baluns but do use 1/4 wave matching transformers of
> RG-11.
>
> Seems picky I know but if someone has an answer I'd like to know.
>
> Thanks,
> Randy
> WX5L 
>
>