Re: Antenna "loading"

Greg Troxel
 

"Michael McEwen mcewenk5osa@... [4sqrp]" <4sqrp@...>
writes:

I am a major electro-dummy...please the elementary question...when reading
a recent QST review, reference was made to the difference in SWR meter
readings based on 25 and 100 Ohm antenna resistance. My MFJ analyzer does
indeed measure resistance in addition to SWR, but I am not sure where it
all fits together. The internet articles I have found are too advanced for
me.
Almost everything in our world is based on the assumption that 50 ohms
is the impedance of transmitters, meters, coax, and antennas. (This
becomes not true when people use open wire line, or 4:1 ununs, etc.)

With a normal radio, an SWR meter, 50 ohm coax, and a 50 ohm resistor at
the end, one should see a 1:1 SWR since the load is matched to the coax
and there should be no reflections. (A key point is that a 50 ohm
resistor while having perfect SWR is a bad antenna.)

A 25 ohm resistor and a 100 ohm resistor should each result in a 2:1
SWR. So in testing an SWR meter, a reasonably easy test is to use good
resistors (that are not inductive or capacitive at the test frequency).
It's common in reviews to measure resistors with both a really good (and
usually expensive) instruement, often lab gear from
HP/Agilent/Keysight/Tektronix etc., and with the device being reviewed;
the point is to check the device being reviewed.

To make an antenna work, it needs to radiate rather than being a
resistor, and you need to couple energy into it efficiently. A dipole
has a nominal radiation resistance of around 72 ohms, even though it's
just wire that has a DC resistance of near zero. So hooking that to 50
ohm cable will result in a decently low SWR (1.3-1.4ish) which will
enable good power transfer.

An actual dipole with other things around it, rather than an ideal
dipole in free space, may have a lower SWR. I have a 40/30/20/17 fan
dipole in my attic and on 40 the SWR is almost 1:1. But surely it's
coupling to other things.

Generally antennas that are full size (dipoles, quarterwave verticals)
will have reasonable radiation resistance and be fairly easy to couple
power into. It's making antennas that are smaller than natural size
work that gets tricky.

An antenna with a radiation resistance of 25 or 100 should accept the
same amount of power from 50 ohm coax. The reason it matters is that
you can use transformers or tuners at the base (4:1 unun is common on a
43' vertical) and there you need to know what it is to try to match it.

Here's an example of a plot of a real antenna from Field Day:

http://www.lexort.com/gallery/random-ham-2015-fd/sark_002

It shows the resistance not quite getting to 50 and an SWR dip. For
actual use this is "wicked good" as we say in the 1st call district.
The plot is made with a SARK 110 that measures resistance as well as
SWR; perhaps the device whose review you are asking about.

73 de n1dam

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