High Power Remote Tuner?

[W8JI]

I don't know if this adds anything to the discussion, and it's probably obvious, but .....

With constant feedpoint power, if you watch the current distribution as you change frequency slightly above and below 9.1MHz you see that the trap current increases with decreasing frequency; however the equivalent series resistance falls with decreasing frequency at a faster "equivalent rate", resulting in less power being dissipated.

The opposite happens with increasing frequency: the trap equivalent series resistance rises, but is offset by the falling current.

At 9.1MHz the I^2R product peaks .......  but then we already knew that !!!

Steve G3TXQ

You can simplify the system into an ~8 foot wire base loaded with a 10  j1200-1300 ohm reactance over perfect ground. It doesn't have to be a trap to have the loss problem Dan found.

:-)

 

[K3VAT]

A trap might have troublesome higher resonances on other high frequencies because of coil self-resonances or the whole antenna might have problems because the interacting set of traps and elements has some additional collective resonances where a lot of current can flow through one or more traps.  It's possible to put those outside the design bands of a trap vertical but still have them close to some ham band.  So maybe 40m,20m,15m is perfectly fine but forcing high power on 17m smokes a trap.

I also doubt that a simple LC trap theoretical maximum  loss across the entire HF spectrum is at its resonant frequency when a constant power is applied at the antenna feedpoint.  I think the resonant frequency of the trap is a local maximum for power loss in the trap in a working trap antenna, so that it's useful to shift the trap resonance down a little bit.  But that doesn't necessarily apply very very far from the resonant frequency when the trap will easily pass current and there's a matching network feeding a lot of power to the antenna despite a huge mismatch.

... Dan

Thanks Dan for pointing out the additional factors that one should consider when designing/building/using traps !!  Plus the cited example is indeed very interesting.  I'll be awaiting more replies on this.

73, Rich, K3VAT

[N3OX]

I've tried a few things and it seems like if you install a very high value inductor that "over-loads" an element a whole lot, you're always going to get a current minimum at a distance away from the tip that corresponds to about the length of wire that would be loaded to resonance by that inductor.

Here's the 30/40 trap vertical in question that has such a bad loss problem, for those who don't have EZNEC to look at the model; frequency is 9.1MHz:

http://n3ox.net/files/3040_trap_9100kHz.png

The top portion above the mid-element "current pinchoff" roughly the same amount of wire on either side of the load so about 17 feet... the +j1400 reactance of the trap at 9.1MHz is just right to load that.  

Here's another example of a similar scenario where the loads are just plain coils (which I think Steve and I had discussed in the past):

http://n3ox.net/files/short_long_40m.jpg

So basically in these examples, we could say that the section of the antenna between the current pinchoff and the actual wire tip is acting as a short loaded dipole with really high impedance at its ends, and that's why there's a current minimum a certain distance away from the antenna tip.

With the vertical, as you go down in frequency, the "current pinchoff" moves toward the feedpoint:

http://n3ox.net/files/3040_trap_8800kHz.png

When the pinchoff hits the feedpoint, the antenna has a very high impedance resonance, more than 5400 ohms resistive around 8.691MHz.  As you go down further in frequency, the feedpoint impedance becomes inductive and the current increases lowering the resistance, until you hit the low impedance design resonance in the 40m band, where the current is about uniform below the coil.

=============

For what it's worth, you can easily scale this antenna model to a nice working 20/15m trap antenna a bit over 14 feet tall with a 1.95uH inductor (kept 0.7 ohms resistance) and a 30pF cap in the trap.  If you do this, it's a good example of a nice low loss 20/15 antenna with a severe loss problem on 17m.  The loss problem does not peak inside the 17m band but there is a lot of loss. If you had a tuner that could feed legal limit to it on 17, you'd dissipate 600W in the coil.  It would have about 7kV across the trap for good measure ... in other words, it's a goner.

[G3TXQ]

You can simplify the system into an ~8 foot wire base loaded with a 10  j1200-1300 ohm reactance over perfect ground. It doesn't have to be a trap to have the loss problem Dan found.

Tom,

I agree!

But what that simplified load wont show is how quickly the trap loss changes with relatively small frequency shifts. The equivalent series resistance of Dan's trap goes from 10 Ohms @ 8.6MHz, to 112 Ohms @ 9.6MHz, and is 23 Ohms @ 9.1MHz.

Steve G3TXQ

[W5WSS]

Kf6bv hello, yeah that could work fine.

The remotely controlled approach that facilitates physically adjustable vertical length is IMHO where one should focus ones efforts.

Unless the half wave fulfills some narrow niche of operating goals, I do not think operating a vertical beyond the quarter wave length per band; contributes enough contact success to justify the problems to overcome with respect to feedpoint auto tuner network pressure imposed by the 1/2 wave family of end fed verticals.
Let alone theoverall system sensitivity to lightning ....it can be done but just say in.

Suppose one can remote control the lengthening and shortening of the element operated against a ground mounted radial counterpoise sufficient for 40m and up would be better investment.

Yes one can simply purchase this style of vertical not promoting anything but the science here. or.

Build it and very gratifying indeed!

I know that you are thinking why did he even mention a half wave?

The reason the trap verticals work are found in the ease of combining a trap that works with a less problematic vertical element length and really a better economical and practical solution.

That is not to say there are no further  advances within the science of antenna development.

As can be seen where N3OX has found some evidence to suggest that problems arise with trap vs  element combinations to avoid we keep striving for perfection and that is a lofty yet worthy pursuit.

I just think for now, we can go with what works and offers better reliability and less replacement costs in field environment network tuners.

There is a place for the vertical family of antennae where we get low enough in frequency that a horizontal radiating dipole needs be so high that a ground mounted vertical becomes a better performer and easier to facilitate.

[N8CMQ]

    In my endless search for ham radio perfection, I am thinking about remote tuning.  The station antenna is a 5BTV ground mounted up the hill behind the house.  It has approximately 50 radials ( actually, I lost count .   I have made many many trips up and down that hill ( oh my aching knees ) setting up and tuning that antenna, and it works pretty well.  I use a Heath SA2040 in the shack to touch it up.  Sure would be nice to have some sort of tuner out there with the antenna, and not separated from it by a hundred fifty feet of coax.
                                                   - Jerry, KF6VB

Hi Jerry!
  I am confused, which is normal for me, but what are you trying to accomplish?

 I have a modified 4BTV with an 80 meter kit that has less than 2:1 SWR, edge to edge,
on the 10-40 meter bands, and the 80 meter resonator is set for the phone portion of the band.

 The instructions for my 80 meter kit advises not to use a tuner to operate the kit beyond the SWR limits,
as it would not survive.

 If your antenna is tuned and the SWR is below 2:1, there isn't any need for a tuner.
The amount of power lost by the frequency/antenna/coax mismatch isn't worth the effort.
If you can't get better than 2:1, then there are other problems.

 What kind of soil conditions do you have? Perhaps you need more radials.
I have 125 30+ foot radials under my vertical, and it performs flawlessly.
Contrary to what some people say, longer and more radials are better,
then you know your power is being radiated and not heating the earth.

If you are trying to tune in other bands, you may want to modify your antenna
with added wire elements in parallel to the trap vertical for those bands,
rather than tune it outside the design parameters?