|[AMRadio] antenna tuners|
lhwill at verizon.net
Thu Apr 20 09:57:03 EDT 2006
Commercially, antennas are built for the task and pattern, match follows.
If you think about it, one cannot obtain resonance of a straight
dipole at every frequency in 75-80 meters (or 10 meters for that
matter). By the equations (there are 3 definitions of resonance by
the way) only one frequency can be "at resonance". So even on 75
while you may cut an antenna say for 3880, it is no longer "resonant" at 3510.
Without getting into the calculus and Maxwell's equations, the
radiation from an antenna is determined by integrating the
incremental current along the wire or conductor. That and only that
determines radiation. Matching is used to reduce losses in the feed
system and or the final tank and to provide a symmetrical load to the
final amplifier. By symmetrical, I mean the R is matching the plate
Z (or xistor Z) and the x varies smoothly + and - around zero as the
frequency is changed from low to high though the channel of
interest. There are many AM broadcast stations of 50 kW that use a
5/8 wave radiator with a suitable matching network. This antenna is
not resonant by definition but nevertheless radiates quite well and
has a gain over a 1/4 wave resonant antenna working against a 1/4
wave ground system. A single frequency T network is used to bring
the input Z to 50J0 at the "doghouse" at the base of the tower. A
properly designed network will have a loss of only a few percent
which is insignificant. (In Broadcast AM directionals, the FCC
"allows" 8 percent loss for ALL the networks, i.e. the phasor/power
divider and each of the tower networks. 8% in power is 0.36
dB!). The only real "problem" and it probably isn't a problem is
standing waves on the feedline will also radiate distorting the
"pattern" but most HF antennas are way too close to the ground anyway
and that really distorts the pattern and greatly increases ground losses.
Many years ago in QST there was an article by an engineer from I
think RCA showing that losses in feed systems at HF are
insignificant. From my Sig Corp texts and others, there are in fact
ground losses associated with any feed system especially if the area
below the feeders does not have a ground screen of some sort and the
balance is not perfect. However, these losses are insignificant
unless the feedline is very long.
The problem with the typical SWR meter is you can't tell what you
have at a dip. The X component polarity cannot be determined so you
are working in the dark. The dip may be Z with the wrong value of R
combining in quadrature with the X to give a Z of 50 ohms but not an
R of 50 ohms. Only a good RF Z meter that displays X and R
accurately can tell you what is really going on. They just don't
exist for balanced lines. If you can truly float the bridge and RF
source and detector you may be able to measure a balanced line at
least at the lower bands. I have RF ammeters in both feeders so I
know that at least I am in balance.
So don't worry about resonance but do worry about baluns with
reactive loads and poorly designed tuners that have high circulating
currents within causing the losses we worry about AND get the wire up
as high as possible unless you are working NVIS.
At 09:06 AM 4/20/2006, you wrote:
>Thank you Brian for your correction.
>Of course we need to achieve resonance for the antennas of solid state rigs.
>And yes they are matching networks, however they do consume power
>that is wasted, especially at the end of a coax. However, they can
>be used at the feed point to much better effect.
>I do honestly believe in using resonant antennas. Also, I did not
>take into consideration the fellows using open wire feed lines to
>baluns inside the antenna tuner.
>I stand corrected.
>However, I will always try to achieve resonance in my antennas as
>that is the best way in my mind to radiate the most signal.
>I agree with you that there are some of us who have to resort to
>tuners for space reasons or other confinements.
>Thanks for assisting me in getting a clearer point across to the group.
>Yes, I do see the need for a matching transformer in some
>situations. I am currently running a tube station, so it can take
>SWR, but I wish to strive for the best match. And yes, the antenna
>tuner is a matching network consisting of coils and capacitors.
>Perhaps I chose the wrong analogy.
>You have provided me with stuff to read up on in order to remind
>myself of lessons learned long ago. Thank you.
>Brian Carling wrote:
>>>I don't believe in them.
>>>If you can't resonate your antenna, then what are you doing?
>>You ar etransmitting some energy and you are getting a whole lot of
>>reflected power that is wasted. You might also be cutting
>>back a lot of your power in the final RF amplifier of a modern rig.
>>>If you use a tuner, you are creating a voltage divider effect that
>>>creates a reactive load in your shack, to ground that makes your
>>>antenna "feed line + radiating elements".
>>Maybe - what if the tuner is outside at the base of the antenna?
>>>Over the years, the importance of having at least a 2:1 match with
>>>at least 1.5:1 some where in the the antenna design.
>>Over the years many of us have transmitted with a 3:1 SWR and no
>>tuner and made plenty of contacts - tube finals of course!
>>>Proper antenna design for your favorite frequencies is the best choice.
>>But some of us can't always get what we want. When you MUST
>>compromise, there is a LOT of good that can be said about a
>>matching network for one's antenna. It is usually an impedance
>>transformer rather than a "voltage divider."
>>>I currently use a multi-element dipole to cover 80,40,20,15 and 10 meters.
>>>Just like microphones in the sound work I help out with at church.
>>>If they are not singing into the mic, I cannot "fix it" without
>>>creating a gain structure that is sucking up to feed back"
>>Different entirely from an antenna tuner.
>>>You need to fix your problems at the source.
>>The antenna is the load.
>>>I our cases, it is resonance of your antenna, by some means.
>>>Even my hamstick on my jeep
>>I have had GREAT success RESONATING antennas with a matching
>>network. I don't understand why anyone would "not believe in"
>>something when they are so effective.
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