[TowerTalk] 80 meter wire beam

[leewells@lexmark.com]

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!).


73's
--Lee - KT4ZX
Georgetown, KY




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