Ken,

There are numerous different overall antenna lenghts which work as well as
each other. There are also a variety of different L/C non-critical values.
The only difference between them is the feedpoint impedance on the lower
frequency band.

But there is only one L/C value which works with each overall antenna
length. And vice-versa. And it must be the right one.

The distance of the traps from the feedpoint is determined simply by the
frequency of the higher band.

If the overall antenna length is arbitrarily predetermined then you have to
calculate the L/C ratio and coil and capacitor values which depend on
antenna length in a very complicated manner. Unless you are very lucky you
will never find the proper values by experiment.

If you arbitrarily predetermine the L/C value and hence the L and C values
by guessing or copying somebody else's antenna, then the correct overall
antenna length is easily found by pruning it in the usual manner. The only
disadvantage is that the antenna length may not fit nicely into your back
yard.

The set of simultaneous equations needed to select the L/C ratio and at the
same time fitting the antenna into your back yard, can be solved without you
being aware of it by the small, self-contained program TRAPDIP obtainable
from website below. Download TRAPDIP in a few seconds. Not zipped-up. User
friendly. Run immediately.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........




"Ken Bessler" wrote in message
news:Tpzhd.40784$lp6.34766@okepread01...
The ARRL antenna book describes the inductive effect
of traps on lower freqs and how this physically shortens
the antenna. They talk about L/C ratio and give two
examples.

Problem is they don't give the formula! #$%#%$!

Here's my antenna - it's a 75/160 trap dipole. The traps
were designed with W8VVM's software. Cutting the
75m portion of the antenna is easy - I need to know how
long (as a starting point) to cut the 160 m end pieces!

Help!

Details: the traps are 8-1/3 turns of rg58 coax wound
around a 3-1/2" form. L is 7.211 uH, C is 232.15 pF.
X is 176.24 ohms. Design freq on 75 is 3.890. Target
freq for 160 is 1.925. I plan on cutting the 75m wires
60.5 feet to start and work up from there but what do
I cut the 160m ends to start?

Ken KG0WX