80/160 meter experimental antenna working at Rattlesnake Island!

[steve_qix]

A while ago (maybe quite a while ago - I don't remember exactly when), I talked about the possibility of building an antenna that would cover 70 and 160 meters.

I am space restricted, so a fan dipole was out.  I can't use open wire line (long story, but it is not an option).

So, the proposed solution was to put high voltage relays at the ends of the 75 meter dipole, and switch in an extra 65 feet at each end, hanging down vertically.  I had tried the 160 meter solution hard-wired so to speak (no relays), and it worked just fine.

So, last year (or was it the year before) I bought a couple of 10kV vacuum relays and some hermetically sealed electrical boxes in which to place the relays.

Well, today I finally installed this setup.  So far, my experiments on 75 and 160 yielded great results.  Everything tunes up properly, and nothing seems to be heating up.

The relays are fired by feeding 24VDC up the coax.  The DC path is up the coax and out one of the wires and to the first relay coil.  The other side of the coil is connected to a 2nd wire that runs right along side the antenna (is lightly wrapped in fact) - broken at the center with an RF choke and the to the other end of the antenna to the other relay.  The path is completed by the 2nd antenna wire, and the other coax conductor.

There is an RF choke at the feedline to isolate the RF from the DC, when DC is fed to the relays.

So far, so good!!  I'll be testing later this evening, and tomorrow (Tuesday) on the Grey Hair Net (I am net control that night).

All tests were done at the pair of 4-400s power level.

We'll see how everything does over time.

Thought I'd report back what happened.

Regards, Steve

[n1ps]

I'm downwind a few miles (well maybe more than that)....I'll report if I detect any smoke in the wind

Hope the summer on the lake is going well.  I'll listen tonight for a strong signal from the west....

p

[steve_qix]

Hi Pete,

So far, so good!  I'll be on tonight with the Grey Hair Net (net control in fact), so that will be a good smoke test of the system.

There is one thing I was unable to get a good handle on and that is the impedance at the end of a center fed dipole.  I just don't know what it is.  I estimated at 6000 ohms after doing some research on the subject.  If that is a good number, the 10kV vacuum relays should be sufficient for the power and modulation levels involved.  I've end fed half wave antennas in the past, and don't recall incredibly high voltages - high, but not thousands of volts worth at least at a few hundred watts.

Certainly an interesting solution to the problem 

Regards, Steve

[steve_qix]

Hmm.  that's very interesting about the traps.  Personally, I have zero experience with trap dipoles.  I would think a 10kV capacitor would have been sufficient, but maybe something else was going on.  Did the caps fail when the traps were in "open" mode, or when operating, say, on 160 meters where the traps were carrying current?  I suppose it's also possible that there was some high circulating current in the traps - or somehow a very high peak voltage was developing due to the resonance.  That is very odd to be sure.

With the antenna at the island, on 160, the relays are closed so there isn't any voltage across them.  But, On 75 - that's another story of course, and since the relays are at the end of a dipole when they're open, and that's where the high voltage comes in.

The question I couldn't answer is what is the impedance of the 75 meter dipole at the ends.  Doing a bunch of figuring and reading, I came up with approximately 6000 ohms.  But, I can't exactly verify the number.

Based on 6000 ohms and the power involved (including the modulation at the highest positive peak the system will develop - 200% positive), the worst-case peak voltage across the relays comes out at around 7000 volts.  If the impedance at the end of the antenna is 10000 ohms, the voltage comes out to be around 8100 V.

I think it's right, but if anyone sees an error or something questionable, please let me know!

Regards,  Steve

[w1vtp]

On the traps: I have the smoldering remains of an 80 / 160 trap dipole in the back room..  It was such a traumatic experience, I took it down, coiled it up and left it in a heap.  I may never do a post-mortem on it.  When it blew the results were quite spectacular.

Good deal, Steve on your version of the dual band dipole.  I'm still wishing I could go on 160 but it seems just out of reach.

Al

[steve_qix]

Good point on the relays and the current.  They are 10A relays so it *should* be enough for the mid-point on the antenna.  Actually, if I think about it, even at the center of the antenna at the high current point, the relays current of 10 amps works out:  (10 squared) times (75 ohms) is mondo power (way over my capabilities) so all should be OK no matter what.

When the relays are open, only the normally open contact is connected to the non-connected wire.  All else is connected to the antenna itself.  When the relays are closed, everything (coil and all contacts) are operating at the same RF potential, so there should never be an arcing problem when the relay is energized.

I am still very curious about the failure mode of those trap dipoles.  Al reported a similar problem, and there have been many other failures reported as well.

[w1vtp]

<snip>
I am still very curious about the failure mode of those trap dipoles.  Al reported a similar problem, and there have been many other failures reported as well.

Steve

You're welcome to do a post-mortem on the ant.  They are still in the "back room."  Funny thing, I used the 40 / 80 meter version for decades without problems at high power.  Maybe it was just that I cut the wire for the wrong length

[ka4koe]

Excelsior!!!!

Well done, lad, well done!

FEELEEP

[N2DTS]

I used to blow out traps at dx100 power level all the time till I gave up on anything but wire in the antenna.
A vacuum relay may hold up since if it arc's it will not make a carbon track.

[steve_qix]

There seems to be something about traps - I've not heard of a single trap dipole that really worked efficiently.  Maybe the relay idea is not so bad!

I did some other reading.  Apparently my figure of around 6000 ohms at the end of a dipole is pretty close at least for antennas less than 1/2 wavelength above the ground.  The ground effects temper the impedance variations along the wire that would occur with an antenna in free space, where the end impedance can get very high.

So, no arcing should ever occur in this antenna with the relays in use, even with a really big transmitter so that's good     The only change I need to make is to increase the number of cores in the inductors - one located at the center of the "extra" wire (to power the relays), and one located at the feedpoint to isolate the RF from the 24VDC supply.

The voltage across any individual core is too high, and the cores get warmer than I would like.  I'm using 4 FB-43-1020 (type 43 material, 1 inch high high and in diameter), and the voltage across each core is too high - around 55 or 60 volts for each core at carrier.  8 cores would be more appropriate, I believe.

Regards, Steve

[AB2EZ]

Steve

In this application, with only a few tens of mA of DC required to operate the relays...vs amperes in class E transmitter applications... I think it would be equally effective to increase the number of turns. I've made RF safety chokes this way. My Ranger's safety choke employs six turns on a single core.

For a given RF voltage across the choke, doubling the number of turns will cut the time varying component of the B-field in half, and also cut the time varying component of the H-field in half (maybe to a value that is a little less than half, due to core saturation).

The DC H-field and the DC B-field will each double... because of the fixed DC current needed to activate the relays, and having twice as many turns... but these are going to be small in any event... and will not have a significant effect on core saturation or core heating associated with the hysteresis losses produced by the time-varying fields.

The net result will be a reduction in core heating, due to hysteresis losses, by a factor of at least 4.

Increasing the number of turns by a factor of 3 will result in a reduction in core heating by a factor of at least 9 in this application.

Stu
 

[steve_qix]

Hi Stu,

The current is 200mA.  These are 12V relays, and the coil DC resistance is 60 ohms per coil and there are 2 in series (24VDC is used).

I wonder if I could do something like 10 turns.  I have 5 turns now running through 4 FB-43-1020 cores.  I used wire with fairly thick insulation to handle any possible voltages that may occur.  It should be less than 1000V under all conditions, but could peak at more than 500V at the edge of the antenna's resonance.  I may be able to get 10 turns through the cores.

Does this sound reasonable?

Thanks and Regards,

Steve

[AB2EZ]

Steve

Since these are the same cores that are used to make transformers for 375 watt push-pull class E transmitters... that operate with 5A DC x 1 turn = 5 ampere-turns in the primary of each phase... then 200mA x 10 turns (2 ampere-turns), should be okay.

I.e. the largest DC H-field in each core is given by: the number of ampere turns passing through the core... divided by the smallest magnetic path length around the core, that includes the current-carrying conductors (i.e. pi x the core i.d.).

So the maximum DC H-field in the core should be less in this application than it is in the class E transmitter application... even with as many as 20 turns.

As an aside: since the highest voltage will be between the first turn and the last turn... that's where you may want extra insulation.

I think that you have the four cores arranged as four individual chokes, in series. If not, I suggest that configuration, rather than running each turn through all four cores. With four individual chokes in series, you don't have the full voltage between any two turns on the same core.

Best regards

Stu

[steve_qix]

I made some changes to the inductors.  Added turns and added cores.  Seems to be running a lot cooler!

This antenna seems to be working out quite well so far.

[steve_qix]

I think that you have the four cores arranged as four individual chokes, in series. If not, I suggest that configuration, rather than running each turn through all four cores. With four individual chokes in series, you don't have the full voltage between any two turns on the same core.

This arrangement seems to be working quite well!  I didn't have enough HV wire to wind the choke the conventional way, so I took your suggestion and set it up as 6 chokes in series.  No problems whatsoever with any H.V. (although the voltage isn't THAT high - still, better to be safe!).