Bandswitched antenna sanity check

[steve_qix]

Ok, all attempts to put up a fan dipole at Rattlesnake Island for 160/75 meters have failed due to numerous physical, space, tree, power line and other factors, so I am left with no apparent alternative other than to make the antenna itself bandswitched.

Note:  I cannot use open wire line at this time - long story, but that is the current state of affairs.  The Island offers a number of challenges, and suffice to say - at least for now, coax is the only alternative.

So, this is what I am going to try:  

1) I have 2 12V coil, 10kV vacuum relays mounted in weatherproof, sealed PVC electrical boxes that will connect to each end of the 75 meter dipole.  The relays, when energized, will switch in an additional 60 feet of wire to each end of the 75 meter dipole, turning it into a 160 meter dipole.  The relay stand off voltage seems adequate for a KW of fully modulated AM.

2) Getting power to the relays - the idea here is to run an extra wire up along side the shield of the coax to feed 12V between the coax shield and the wire.  Wire is electrically shorted to the coax with a suitable capacitor at the center insulator.  The coax (and the extra wire) is somewhat electrically isolated from the antenna itself using about 20 type 43 ferrites.  I could also insert a suitable capacitor in series with the coax at the antenna change over relay, and (through a 2nd choke) feed DC up the coax.  With this method, 2 chokes are required at the center insulator to get the DC to each side of the antenna.

3) Each antenna wire will consist of 2 conductors to facilitate getting the DC from the center of the antenna out to the relay coils.

4) ------>>>  This is important  - I wound a 270uH choke using 4  FB-43-1020 cores with 5kV H.V. wire.  My calculations indicate about 800V peak (RF) could occur across this choke under the most extreme positive modulation the transmitter can develop, operating at full power output.  The choke is there to transfer the DC from the extra wire (which is on the shield side) to the other side of the antenna (the center conductor side) and at the same time blocking the RF.  The choke will electrically appear across the antenna itself.  270uH seems like a reasonable value for a 50 or 75 ohm system.  The inductive reactance is high as compared to the antenna impedance.  Does the choke design seem reasonable with respect to the voltage across it, and the inductance?  The coil DC current is about 100mA so DC saturation should not be an issue with the (I think 7) turn choke.

Yes? No? Maybe? Crazy? Anyone else ever try this?

[KA2DZT]

Steve

What other ideas can you come up with??

I think a trap type dipole would be a better way to go.

Fred

[KB2WIG]

S,

Crazy does help. Worked for me all these years.

Would you care to expand on why the fan will not work? I can't figure out why when adding to the 80m dipole to make it a 160m dipole, hanging a new 80m dipole under it will not work. Or is it a question of the visual impact on the island (not the 'eye candy')?

I've had some sucess with my 80/40m fan.


klc

[W3GMS]

Steve,

Oh yes, the challenges!   Your proposed scheme, I am sure could be made to work, but its a lot of work to implement.  Here is a scheme I would try and although it may not be quite as effective, I think the results would be acceptable especially when compared to the alternatives. 

I would go back to the fan dipole but it appears your having  problems with the space issue of having it full size and the worse case band for the size issue would be 160M.  So assuming you have enough space for an 80M antenna then use loading  coils in each side of a shorter wire for 160M.  Use the same fan scheme just shorten the physical length and use the coils for 160M.  I prefer that over a true trap approach since with traps you would suffer bandwidth issue on both bands.  If your full size on 80 no BW issues there and on 160 I am sure the Q would be higher but your operation centers mostly I believe around 1.945.   A tuner tweak I  am sure would allow acceptable performance on say 1.885. 

Joe, GMS

[AB2EZ]

Steve

I don't know what problems might arise with your proposed design... although the standing wave patterns are such that, at the ends of a dipole, the RF voltage between the physical wire and anything that is nearby and has an RF conducting path to ground will be a lot higher than the voltage across the 50 ohm coaxial feeder cable. The 60 foot, unused (on 80m) add-on wires will be capacitively coupled to ground, and therefore the RF voltage across the open relay contacts on 80m may be a lot higher than you think it will be.

As an alternative (not necessarily better than your proposed approach), see the attachment:

Consider using a folded dipole version of your proposed approach. You can use capacitors at each end (anywhere beyond the relay) to block the DC path. In 80m position, the relay will add a capacitor across the two wires to complete the RF path around the shorter pair of wires that comprise the 80m folded dipole. On 160m, the relay contacts will be open, and RF will flow around the full length folded dipole. With two wires making up the folded dipole, you can feed DC to activate both relays via the two wires of the feedline (no need for an extra external wire). You can use a balun at the feed point to match the folded dipole to the coaxial feed line... and you can use capacitors and RF chokes at the feed point to prevent the balun from shorting out the DC; and to couple DC, around the balun, from the coaxial feed line to the folded dipole.

The advantage of this approach (if there is one) is that you don't have very high RF voltage across the relay contacts, and you don't have to try to block the RF current along the wires that carry the DC to the relays.

I'm not sure whether or not there will be any "common mode" current flowing past the locations of the relays on 80m (when the relay contacts are closed)... and if there is, this may have some effect on the dimensions of the center section of the antenna, at resonance, on 80m.

Stu

[W2NBC]

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[AB2EZ]

Steve

I did a quick simulation of my suggested switched folded dipole using EZNEC, and it appears to have a very high SWR on 80m (relay contacts closed). It behaves like a dipole antenna that is twice as long as it should be, even though half of it is a folded dipole and the remaining ends are essentially a regular dipole.

To make it work, you would have to open the connections to the extended ends (both top and bottom) on both sides... as well as adding in the shorting wires between the top and bottom on both sides.... so it doesn't seem to be a good approach



Stu

[steve_qix]

Hi Stu,

Thanks for doing more research.  I have 10kV vacuum relays at the end of the antenna.  The only concerns I really have are:

1) Will the chokes be sufficient.  I used 4  FB-43-1020 cores to wind an approximately 270uH choke.  The voltage could be as high as 600V or thereabouts on absolute peaks.  Each core will be subject to about 150V each, on peaks.

2) Will the voltage at the ends exceed the relay breakdown voltage.   I did some research, and from what I could gather, with 800 watts of carrier, modulated to 180% positive, I came up with less than 10kV - about 6kV on absolute modulation peaks, as I recall.  However, I do not know how to actually calculate the value since I don't know the actual impedance at the end of the antenna.  From what I could gather, the impedance at the end of a 1/2 wave dipole seems to be around 5000 ohms, which works out to be about 6000V on peaks.

Anyway, that's what I'm concerned about.

I have net control of the Grey Hair Net on this coming Tuesday.  If the antenna at the Island isn't 160 meter capable, I'll have to come home and run the net.  Oh well!!

[AB2EZ]

Steve

Please see the attachments.

The extra horizontal wire will be coupled to the main horizontal wires of the dipole from the center conductor to the location of each relay. You need to break the RF path on the extra horizontal wire* with a choke at the feed point... just as you need to use a choke to couple the extra DC feed wire to the right side of the antenna. Therefore, I think you will need two chokes. The peak RF voltage across each choke (i.e. the peak of a single cycle of RF during a positive 180% modulation peak) will be around sqrt (2.8 x 2.8 x 800W x 2 x 50 ohms x SWR) =  792V x sqrt (SWR). SWR is the actual SWR on the feedline at the frequency of operation. This is a little higher than you calculated.

*I did a quick check with EZNEC, and having the extra horizontal wire, parallel to the main dipole wires, with an unbroken RF path end-to-end (but not connected to the main dipole wires) appears to result in a significant increase in SWR (i.e. a significant increase in driving point capacitance/negative reactance)

At 1.8MHz, 270uH corresponds to about j3054 ohms of reactance... which is more than sufficient but:

Will the choke be able to handle the peak RF voltage across it? [i.e. the peak voltage = the number of turns x the cross sectional area of the core x 2pi x the RF frequency x the peak value of the B field. The peak B field (permeability x the H field) must not saturate the core material]

Based on my experience in using 1 core with 7 turns to make a safety choke for the output of my Ranger (it doesn't get warm on 80m), I think you might be okay (more turns x more core cross sectional area => less peak B-field, for a fixed peak voltage and a fixed frequency)... but perhaps you should check it by putting it across the output of your transmitter at home. If your design is marginal, the cores will get very hot, very fast. You might need more turns, even with the DC required for the relays.

Will the peak RF voltage (with respect to ground) at the endpoints be less than 10kV?  Using 5000 ohms as a working impedance level, my calculation is that the peak RF voltage (i.e. the peak of a single cycle of RF during a positive 180% modulation peak) will be around 7920V... so, you might squeeze by. Perhaps you should be a bit more conservative with modulation peaks until you gain some actual experience. Actually, I haven't yet found a way to calculate this voltage that I have confidence in. If I had to pick a conservative rule of thumb, it would be 10x the voltage across the feed point of the dipole. I.e. 10 x the RF voltage across the chokes. This rule of thumb gives the same answer as you get by looking at the formulas for an end-fed half wavelength dipole, and asking what the peak source voltage would be if the dipole were driven from one end (with a long counterpoise). http://www.aa5tb.com/efha.html. It is also the same answer you get by scaling the impedance from 50 ohms to 5000 ohms with a fixed power level (as above).

Good luck
Stu

[steve_qix]

This is good information!  I will do some testing.

Yes, 2 chokes are definitely needed.  My thought was to put the 2nd choke at the transmitter (T/R relay) end to avoid having any more stuff/weight up in the air than necessary.  So, in that case the DC path would be through the center conductor of the coax, etc.  Also, in this way, I can see if the choke is getting warm since there will be identical chokes at the T/R relay, and at the center insulator.

I really appreciate the thought on this project.

Regards,

Steve

[AB2EZ]

Please note:

I have updated the last paragraph of my previous post. I have also updated the attachment to make it simpler/easier to understand.

In particular, I now suggest that if I had to pick a conservative rule of thumb, for calculating the voltage between each end of a half-wavelength dipole and ground, it would be: 10x the voltage across the feed point of the dipole. This rule of thumb gives the same answer as you get by looking at the formulas for an end-fed half wavelength dipole, and asking what the peak source voltage would be if the dipole were driven from one end (with a long counterpoise). http://www.aa5tb.com/efha.html. It is also the same answer you get by scaling the feed point impedance of the antenna from 50 ohms to 5000 ohms, with a fixed power level.

Stu

[steve_qix]

Just to follow up    This project is still pending.  A lot going on at Rattlesnake Island this year (a new building, to be exact!).  Hopefully, I will have time to construct the antenna and test it, as described    The vacuum relays are mounted in waterproof boxes as of this writing.