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1-8 of 8 messages
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Shunt feed a tower - Simple instructions please?
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by KB7TEF on October 17, 2005
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Hi, I have a USTowers 555 HDX tower with a Force12 C3s and an EF240 ES antennas, and a Diamond 2m/440 on top. I have read a lot of stuff on shunt feeding, but can't really find a step-by-step way to go about it.
I would like to use the tower for both 80m and 160m, since wires I put up tend to couple with my two beams. I do run 1200 watts frequently on lower bands (have to with the lousy antennas I have), so the match needs to be able to handle that.
I have been told I need vacuum variable capacitors, but when I look them up on the net they are $150 plus. Is there something more economical or another way?
I would love to see step-by-step instructions for someone who has never done this before. And, if you have a shunt fed tower, do you like it? Performance?
Thanks in advance!
Steve (now N1VL)
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RE: Shunt feed a tower - Simple instructions pleas
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by WW5AA on October 17, 2005
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Steve,
First of all I hope you understand that shunt feeding a tower is just like any vertical in that you need at least 60 radials( and more like 120) before it will compete with a trap vertical or vertical dipole, assuming the same gounding. The tower can be fed several ways and there is no easy fix. In my opinion only (yes I have shunt fed a tower),unless the tower is at least 100" and has a good radial system it is not worth the time and money (a whole lot more than $150). I would also like to see a plan that would work and hope someone will enlighten us.
de Lindy
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RE: Shunt feed a tower - Simple instructions pleas
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by W5GA on October 18, 2005
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TEF, your tower is too short to make this really practical on 160. Per the ARRL Antenna Book, a beam on the order of a TH-7 allows shortening of the tower by about 40' by adding capacitance. That would make a 90' minimum. This is theory only, as I only know what I've read. I'm planning on doing this with 130' of R-25 (as soon as I gather the requisite tower sections).
It should work ok for 80, thought. YMMV!
It will be interesting to see what the guru's think!
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RE: Shunt feed a tower - Simple instructions pleas
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by WA4PTZ on October 18, 2005
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I shunt fed my 50 foot tower and have seen a 35 foot
tower shunt fed. You can create artificial length
by using a wire from the tower to an external
connection via a variable capacitor. There was an
article on this in QST back in the seventies.
If you have no other choices, go for it, but a
dipole would work better, even a trapped one.
And yes, you do need radials. The dipole is also
less work.
73 ,
Tim WA4PTZ
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RE: Shunt feed a tower - Simple instructions pleas
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by K4SAV on October 18, 2005
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I'm afraid there are no simple step-by-step instructions on how to do this. Towers with all their associated cables create special cases for every installation. Tower electrical height and radiation resistance determines what kind of matching network you need. That's a function of tower height and all the stuff at the top, and how it is distributed at the top.
Your tower probably uses the coax arms to hold the cables. These cables will be extended away from the tower by 2 to 3 feet. Because of this, they may create some current flow into your rotator circuitry. Some people have lost rotator position pots this way. You need to evaluate this problem and, if necessary, devise a solution.
Your cable shields all have to be connected to tower ground at the bottom of the tower. They also need to be connected at the top of the tower. This will help with the rotator RF problem (if you have shielded rotator cable, - which you probably don't). All cables have to exit at the bottom of the tower.
I would recommend about 60 quarter wave radials.
Your tower with the stuff you have at the top should resonate somewhere around 2.5 to 3 MHz. This should work out well for 80 meters. At 160 meters it is kinda short. The radiation resistance at 160 meters will probably be around 4 to 5 ohms. This makes it very difficult to maintain good efficiency. Ground resistance with 60 quarter wave radials will probably be around 6 ohms. That means the best you could hope for on 160 is about 40 to 45%. It will be less, because other losses have not yet been included. You can match just about anything, but efficiency is what you have to worry about. That said, this antenna on 160 is probably still better than a low dipole (<50 ft) for DX. A low dipole radiates mostly straight up and also has high ground losses. The dipole will be better for short distances.
For the matching network, you need to do some reading. The network will be different for 80 and 160, and some switching will be needed. You need to do some design work. If you have an antenna simulator program such as EZNEC this would be a big help. If not, use some examples, that are for tower lengths close to the same you have, from the ARRL Antenna Book or ON4UN's Low Band DXing Book. You need to consider your power level and the voltage ratings for the components. For matching on 160 you will find there are large voltages on the components, because of the low resistance and high capacitive reactance of the antenna which you are trying to match. You may have to change the type of matching network used, because the voltages may become too high for any practical components. Voltages on the 80 meter components will be much lower. When you use inductors in the matching network you have to pay close attention to Q. You will probably have 1 to 2 dB loss in a good coil used in this network for 160.
You can experimentally determine the component values you need using small variable components and low power. When you have determined the values, then get some components rated for the power level you are using. Many people that run high power use vacuum variable capacitors and vacuum relays. You have to worry about moisture and condensation. You can use air variables, rated for the proper voltage, if you can keep them dry. If you live in an area with significant lightning, and you use any mica transmitting caps, make sure they are not connected to the circuit when the antenna is not in use. A nearby lightning strike will take them out.
Read about the shunt feed wire. It usually needs to be 2 to 4 feet away from the tower. Also a cage wire, instead of a single wire, for the shunt has some advantages. The attachment height will also vary. For 160 it will, without doubt, need to be as high as you can get it. For 80, I'm not sure. You will either have to experiment, simulate, or both. It gets more complicated if you have to do two wires at different heights, plus on your crank-up you only have three choices of heights that you can select.
There just isn't enough space here to give you all you need to know to do this properly. You are going to have to dig for it. You might be able to just throw something together and get it to work at low power, but the results would probably be less than you had hoped for.
Jerry, K4SAV
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RE: Shunt feed a tower - Simple instructions pleas
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by K4SAV on October 18, 2005
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One other item. If you decide to only use the tower on 80 meters, about 32 radials, 0.2 wavelength should be sufficient. Since the tower radiation resistance should be somewhere around 25-30 ohms, and 32 radials should get you about 8.5 ohms of ground loss, antenna efficiency should be 75-78% (before subtracting other losses). This is a big savings in the amount of ground wire vs radials to handle 160 meters.
Jerry, K4SAV
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RE: Shunt feed a tower - Simple instructions pleas
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by WB6BYU on October 20, 2005
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(1) put LOTS of radial wires around your tower and connect
them to the base.
(2) Connect a length of #12 wire to the top of the bottom
tower section.
(3) Mount a remote autotuner in a convenient spot between
5 and 20 feet out from the base of the tower. Connect the
ground of this tuner to the tower base with wide copper
strap.
(4) Connect the #12 wire to the feed terminal on the
tuner. Connect coax from the tuner to the radio and
adjust the tuner in the normal manner.
If you don't have an automatic tuner, you can measure
the impedance at the feedpoint and then build a custom
tuner to match that impedance on your desired band.
For high power use you may want to use copper tubing
for coils and rugged capacitors capable of withstanding
both high voltage and high currents.
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RE: Shunt feed a tower - Simple instructions pleas
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by K4SAV on October 20, 2005
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When you asked for simple step-by-step instructions, I replied there are no simple step-by-step instructions. I also said that you could just throw something together and maybe get it to work at low power, but you will probably be disappointed at the results.
As an example consider the following: #12 wire connected at 22 feet, 4 feet from the tower, and connected to an autotuner. Using 60 radials, each 136 feet long. Using a simulation of your tower and similar antennas on top:
At 160 meters the impedance seen by the tuner is approx. 10 +j120. Most of the R is contributed by 6.7 ohms of ground loss and the resistance of the #12 wire. If ground and wire losses were zero, the impedance would have been 0.47 +j120. Antenna gain is -15 dB with no tuner losses. Tuner losses will be significant. I doubt most tuners will be able to match this. But that really doesn't matter, the antenna is pretty much a dummy load anyway. The reason for this is that only the first 22 feet of your tower are doing any significant radiating. Currents in the tower above 22 feet are very small.
At 80 meters things are much better. With the same radial system, at 3.6 MHz, the impedance is 30 +j210. A GOOD tuner should handle this. Tuner output will be about 1190 volts at 5.9 amps. Antenna gain = -0.45 dB. If you reduce your radial system to 32 radials, each 56 ft in length, the gain goes to -0.5 dB. You still have to subtract tuner losses, but it's not a bad configuration, although some improvement can be made in the shunt wire.
Your tuner has to be located at the base of your tower, or very close to where the tower is tied to the ground radials. If you don't do this you will have large currents flowing back down your feedline.
As I said the first time, the tower is a good match for 80 meters, but it is too short for 160, and the best configuration for 160 is going to be with the shunt as high as you can get it. I did not completely determine what is required to get this to work on 160. It is going to require some work on the shunt feed. A #12 wire doesn't even come close to what is required. It is going to require some kind of large cage wire located much further away from the tower. Once this is determined, then you can figure out if it will also work for 80, otherwise you will need two shunt wires.
A disclaimer: the antennas used at the top in the simulation are not exactly what you have, but close. How you configure the shunt wire (its size and position) has a significant effect on the gain. So the numbers given are not exactly accurate, but they should be close.
Jerry, K4SAV
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