[TowerTalk] 80 meter wire beam
Sat, 01 Nov 1997 00:24:49 +0000
i have one based on the on4un two loop reversable design. it
uses a 1/4 wave feed line to the inverted v element with a
capacitor at the end... transform the capacitor through the
1/4 wave and you are really inductively loading the reflector
to make it longer.
for my plan.
> I'm considering putting up an 80m, reversible wire beam, and
> I'd like to get any thoughts and comments you may have.
> My starting point is the design in ON4UN's book where he uses
> 2 inverted V elements and then capacitively loads one element
> as a director using a piece of coax. The dimensions shown in
> the book say 67+ feet on each side of the V, but through the text
> it talks about a little over 65 feet????
> So here's where I'm headed. 2 Inverted Vee elements, 120 degree
> angle between the legs. Spaced 40 feet apart at a height of 67
> feet. Design frequency is 3.84 MHz, optimizing for max gain. I've
> been using NEC2, and here's the data I've come up with.
> leg length max gain Director Load Input impedance
> ----------- -------- ------------- ---------------
> 60 feet 8.61 dBi +50 (inductive) 22.38-j84.48
> 62 8.64 0 24.84-j38.11
> 63.5 8.64 -60 (capacitive) 30.24+j8.70
> 65 8.64 -100 32.08+j45.95
> 67 8.64 -150 33.28+j97.09
> 70 8.63 -250 39.12+j186.67
> Looks like using the 63.5ft leg length will be best. The actual
> resonant frequency is about 3.824 Mhz (33+j1.33) with a gain
> of 8.57 dBi.
> Max gain angle is about 45 degrees elevation in all cases.
> I'm planning on using 2 identical pieces of coax that are the correct
> length to provide a capacitive load of 60 ohms going to a switch box.
> Of course I'll switch both ground & center conductor.
> ON4UN shows a little LC circuit in the switch box to match to 50 ohm
> coax. I'm not sure what my 30.24+j8.70 will look like after about 20
> feet of RG8X (more calculations left to be done), but I could probably
> use a similar LC circuit.
> Any other ideas on matching??? 1.6:1 SWR isn't bad to live with, maybe
> a network isn't needed????
> So does all of this data sound reasonable?
> I also noticed that going from a flat-top dipole to the inverted
> Vee elements (120 degrees) looses almost 1 dB of gain (OUCH!).
> --Lee - KT4ZX
> Georgetown, KY
> FAQ on WWW: http://www.contesting.com/towertalkfaq.html
> Submissions: firstname.lastname@example.org
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: email@example.com
> Search: http://www.contesting.com/km9p/search
David Robbins K1TTT (ex KY1H)
firstname.lastname@example.org or email@example.com
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com