TopBand: "trapped dipole"

[George Guerin]

--IMA.Boundary.225188648
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     K9JF wrote:
     
>I need some ideas from the collected masses regarding a horizontal 
receiving/transmitting antenna for 80/160.
     
Near the top of my tower at the 95' level, I have an inverted vee for 75/80 
meters with a manually operated stub to change between the portions of the 
band.  When I built the antenna last year, I "cut" the supporting lengths 
past the insulators to be resonate on 160.  However, I would like to devise 
something that would allow me to switch from 80 to 160 without manually 
attaching a stub (it's hard to do this at night when it's dark!).
     
I thought about a "trap" at that point but cannot find any info in the Radio 
Amateur's Handbook, Antenna Handbook, or ON4UN's book.  
     
I also remember back when I first started this madness (radio that is), we 
built dipoles with a single feedline but with multiple lengths of antennas 
(80 meters, 75 meters, 160 meters) connected to the coax.  In fact, HyGain 
use to market this kind of antenna.  I could go this route but would still 
need additional wire for the new dipole.
     
Any recommendations on a workable solution to this configuration?
     
73 Jim K9JF/7
Vancouver, WA
jfenster@hpux.mesd.K12.or.us 
     
     Dear Jim:
     
     We have put traps in inverted L's and dipoles for 80 and 160 before.  
     My personal opinion is to go to a conventional trap with ceramic door 
     knob capacitors and a coil.  Hang it off of or put it around a six 
     inch long "dog bone" insulator.
     
     The approximate values needed are 12 uhy and 170 pfd to resonate the 
     trap on 80.  I recommend two ceramic door knob insulators in parallel, 
     like two each 75 or 85 or 100 pfd caps..  as the current is very high 
     and at a kw, the caps will heat up and detune and maybe blow up!  I 
     have used 3 inch OD B&W coil stock, either 4 or 6 turns per inch.  
     Another possibility is to use UV resistant CPVC pipe as a coil form.  
     The 2-1/2 inch pipe is actually 2-7/8" OD, which will work fine with 
     12 gauge enamelled copper wire.
     
     Dip the coil and capacitor to about 3525.  If you put one in each side 
     of your 80 meter dipole you should get about 30 feet of inductive 
     loading on 160 meters; that is the overall length should be about 
     228-230 feet.
     
     If you want to get out of the switching wire stub mode for 75 and 80, 
     there was an article many years ago about a pair of dipoles on one 
     feed line for 80 and 75.  You could make a new 80 meter dipole 
     perpendicular to the 75 meter wires.  Then you could eliminate the 
     stub.  Also, you could then put the traps between the 75 and 160 
     portions of that dipole, or, if you don't like traps, run a third set 
     of wires in between for 160 meters.  There will be some interaction 
     but, if you can prune, you can save some trap building and have 160, 
     80 and 75 with three sets of wires on one feed line.
     
     Good luck, hope it isn't too cold out your way.  
     
     George, K8GG.
     
     PS:  I, personally, do not recommend traps made of coax.  The only one 
     I read about that looks good is the parallel RG59 article early this 
     year in either QST or CQ magazine.  A single winding of RG 58 or RG 8X 
     will heat up at a KW and eventually fail!  K9AJ's crew at AH1A proved 
     that by inductively heating a CPVC coil form at night while operating 
     a Battle Creek Special on 160 and during the day Mother Nature helped 
     by solar heating it to put a permanent curve in an 80 meter trap that 
     is now out of service on his trophy shelf.
     
     
     Also, I don't know what effect the traps have on your 75 meter 
     resonance, but I do know traps seem to isolate much better above their 
     tuned frequency than they do below.  Hopefully they won't change the 
     tuning much at 3790, assuming you are still switching a stub in and 
     out to go from CW to SSB.
     
--IMA.Boundary.225188648--

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