[HCARC] Antennas, Radios and Elmering
Dale Gaudier
dale.gaudier at windstream.net
Sun Aug 5 17:15:58 EDT 2012
Gary:
No need to buy a fancy SWR analyzer - just borrow one. I have a Rig Expert
AA-54 that will tell you more than you ever wanted to know about your
antenna (and attached feedline). It can be used in standalone mode attached
to your antenna, or connected to a computer which then allows you to
download the data from the analyzer and do some pretty sophisticated data
analysis and graphing. It runs circles around low-end SWR analyzers such as
the ones MFJ sells. Give me a call or send me a private email to set up a
time to meet.
One of the real benefits of this unit is that it will not only tell you the
impedance of your feedpoint, but also tell you the magnitude and sign of the
reactance of this impedance (i.e. whether it is capacitive or inductive).
This makes it much easier to adjust your antenna for resonance and to help
match the antenna to the characteristic impedance of your feedline. This is
covered in any good antenna handbook, such as the one put out by ARRL. It is
also covered by the study materials in the Extra License manual (you were
going to study for your Extra, weren't you? :)
Keep asking questions - it's how we learn.
73,
Dale - K4DG
-----Original Message-----
From: Gary and Arlene Johnson [mailto:qltfnish at omniglobal.net]
Sent: Sunday, August 05, 2012 12:11 PM
To: Kerry Sandstrom
Cc: hcarc at mailman.qth.net
Subject: Re: [HCARC] Antennas, Radios and Elmering
Kerry,
Thank you for the good explanation, and I remember most of that in bits and
pieces from the antenna books. Good to have it in one place in a coherent
narrative. BTW, narratives like these have value and should be saved and
printed for posterity. Who knows - it may save some Newbie from frying his
radio - Dale said smething about the radio losing it's vital smoke or
something like that.
What I get from it also is get an antenna analyzer and put it up and try it.
If it works - great, if it doesn't modify it until it does to your
satisfaction (really to the radios satisfaction) and if it doesn't or both
you and the radio are not happy, sell it on Ebay.
All of which is probably the reason the guys in Mesa AZ get together on a
couple of mornings each month and try out antennas. My offer to buy donuts
for each and every antenna meeting still stands. Who knows, we may find
something that works particularly well in the Hill Country area. If not, we
will at least have had fun and eaten some good donuts and that is not all
bad.
Gary J
N5"BAA"
----- Original Message -----
From: "Kerry Sandstrom" <kerryk5ks at hughes.net>
To: "Gary and Arlene Johnson" <qltfnish at omniglobal.net>;
<HCARC at mailman.qth.net>
Sent: Sunday, August 05, 2012 11:35 AM
Subject: Re: [HCARC] Antennas, Radios and Elmering
> Gary,
>
> There are rules and they indeed are followed. Its not always obvious
> what rules apply in a given situation. It takes years to figure out
> what all the rules are. Actually a college degree does not always
> help. College courses give you the very basics and teach you how to
> extend those basics to real life situations. It doesn't matter what
> your major is/was, its still the same. You have to look at each
> situation and try to figure out what assumptions you can make and
> therefore which rules apply. Actually they all apply, but in many
> practical situations they don't matter. The theory of Relativity is
> an excellent example. If you're driving your car at 30 MPH,
> relativity still applies, however, the difference in the answer if you
> include relativistic effects and the answer if you ignore them is so
> small that no one includes relativistic effects. If on the other hand
> you're a subatomic particle going "mach a bunch", near the speed of
> light, if you don't include relativity you will never get close to the
right answer.
>
> With antennas, there are two main characteristics you should be
> interested in from an RF viewpoit. The first is the radiation
> properties of the antenna and the second is the impedance of the
> antenna where you apply RF to it. These are independent of each
> other. You need to be concerned about the impedance because you have
> to get your transmitter to deliver power to the antenna. In EE the
> maximum power theorem says that in order to deliver the maximum power
> to the load, the impedance of the load (for us the antenna) must match
> the impedance of the source (for us, the transmitter). Sinced our
> transmitters typically, but not always, have an impedance of 50 Ohm,
> our load should also have an impedance of 50 Ohm. At this point we
> have a choice. We can either use an antenna that has an impedance
> that matches our transmitter and connect the two with a transmission
> line that has the same characteristic impedance (called a matched
> line) or we can use any old transmission line and antenna impedance we
> want and use a matching network (called an antenna tuner) where we
> want to change from the 50 Ohm impedance to whatever impedance the
antenna/transmission line has. We need to define 'resonance'.
> Resonance is the frequency where the load or circuit or whatever has
> an impedance that is purely resistive. An impedance in general has a
> resistive part and a reactive part. The reactive part may appear
> capacitive or inductive. A wire that is approximately half a
> wavelength long is resonant. If you break the antenna in two at the
> middle and measure the impedance at that point, you will find it is
> resistive and approximately 75 Ohm. This is called a half wave dipole
> and is probably the most common ham antenna in the world. Since it is
> balanced, the two sides are the same, it should be fed with a 75 Ohm
> balanced line (twin lead or coax with a 1:1 balun). If you don't feed
> it with a balanced line, ie, you use coax cable, you introduce an
inbalance in the system.
> As a result, the feedline , coax cable, will radiate and distort the
> radiation pattern. At HF close to the ground, you will never be able
> to measure this. At UHF you will. Ignore the balance issue at HF.
> This is an example of a rule that doesn't matter so doesn't need to be
> followed at HF.
>
> If you take the same dipole and feed it at the end as W$WJ tried to do
> with his DX-35 many years ago, you will find that since the wire is
> still resonant, the impedance at the end is still resistive , but the
> impedance is now ~1000 Ohms. A 450 or 600 Ohm parallel line is the
> correct transmission line, but what is the DX-35 going to do when it
> sees ~1000 Ohm load instead of the 25 - 200 Ohm load it can handle?
> What the DX-35 does is not deliver very much power to the load. You
> can tell because you can't find a setting of the tune and load
> controls that give the correct readings on the plate meter. If you
> want to use an end fed wire and an open wire transmission line you
> need something to change the high impedance of the load to a low
> impedance that the DX-35 can use. Our new rigs, don't have tune and
> load controls, they have a fixed tune output that needs to see a load of
50 Ohm. It only makes life harder.
>
> The Windom of any variety, uses a feedpoint some where between the
> center and the end. Its impedance is somewhere between 75 Ohms and
> ~1000 Ohms, if its at a frequency where the wire is resonant. If its
> not at a resonant frquency, its impedance will have both a resistve
> and a reactive component. You have to remove the reactive component
> and transform the resistive component to 50 Ohms. That is what an
> antenna tuner does, or in the case of the G5RV and Carolina Windoms,
> what the lengths of transmission line and ferrite transformers/baluns
> do. Since everything depends on lengths of antennas and transmission
> lines in terms of wavelength, as the frequency changes so does the
impedance.
>
> While I have talked about a half wave wire, that isn't particularly
> critical to the discussion. Any hunk of metal will have an RF impedance.
> Depending on where/how you feed it, that impedance will vary all over
> the place. In general, you like the hunk of metal to be a half
> wavelength or more in at least one dimension. At this size it will
> normally have an impedance that is easier to match than extremely small
hunks of metal.
> The shape doesn't really matter. you would like it has high as possible.
> The only way to get practical gain out of an HF antenna is to have
> enough metal in the air to cover a volume on the order of a
> wavelength. The critical measurement is called the antennas effective
> aperture. If you know its effective aperture, you can estimate its
> gain and beamwidth from that measurement.
>
> So Gary, to your specific question, you can put that antenna together
> anyway you want. You have to figure out a way to match the antenna to
> the radio. That includes both physically with a coax connecter/cable
> of some sort and electrically with some kind of matching system. If
> you put the antenna very much higher and try to use it on 20, 15 or 10
> meters it will no longer be "optimized" for NVIS, but it will still
> work. If you want to just plug it into a radio without a matching
> system, i don't it will work very well. If you have an antenna tuner
> and adjust it properly, it will work as well as any othe rantenna of
> similar size. The practical stuff is simple - just put it up and play
> with it until it works. Understanding why and understanding the
> comments others make about an antenna - that's hard.
>
> Kerry
>
>
>
>
>
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