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

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

"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!
[...snip...] Trap data un needed.

Summarizing the other two posts and what you alredy know. With the effects
of the surroundings the formulas won't be much good anyway.

Put the traps at the ends of the standard 75M dipole [["75m portion ...is
easy ..."]]. Make this a little long if U like. Add a little too much wire
for the 160, then measure the 75M and 160M min SWR freq. Adjust the 75M
length for your 75M freq of interest. Then, using 468/F figure out how far
off you appear to be on the 160 length. Trim off about 80% of that value,
then trim-by-SWR the rest of the way.

Sound good fellas?

[[personally, i think if ur 2:1 makes no diff at the other end]]]
--
Steve N, K,9;d, c. i My email has no u's.

"Steve Nosko" wrote in message
...


Summarizing the other two posts and what you alredy know. With the
effects
of the surroundings the formulas won't be much good anyway.

Put the traps at the ends of the standard 75M dipole [["75m portion ...is
easy ..."]]. Make this a little long if U like. Add a little too much
wire
for the 160, then measure the 75M and 160M min SWR freq. Adjust the 75M
length for your 75M freq of interest. Then, using 468/F figure out how
far
off you appear to be on the 160 length. Trim off about 80% of that value,
then trim-by-SWR the rest of the way.

Sound good fellas?

[[personally, i think if ur 2:1 makes no diff at the other end]]]
--
Steve N, K,9;d, c. i My email has no u's.


That's basically what I did. I ran into an interesting issue tuning
the 75m portion - as I cut the antenna in shorter and shorter
increments as I approached my design freq, something wierd
happened - I was at 3.844 and cut 12" off each end which got
me 3.868. I cut another 6" and got 3.860? At that point I stopped
cutting. My traps were designed for 3.890 but I had no way to
measure them.

Why did trimming 6" lower my resonant freq 8 kHz? I'm guessing
but could my traps be closer to 3.86 than 3.89? I'm afraid to cut
more.

Here are the results I got:

Band 2:1 SWR range Bandwidth Center Best SWR
160 1.862-2.040 0.178 1.951.0 1.0:1
80 3.759-3.916 0.157 3.837.5 1.0:1

Ken KG0WX

"Ken Bessler" wrote in message
news:jEZhd.41398$lp6.36622@okepread01...


That's basically what I did. I ran into an interesting issue tuning
the 75m portion - as I cut the antenna in shorter and shorter
increments as I approached my design freq, something wierd
happened - I was at 3.844 and cut 12" off each end which got
me 3.868. I cut another 6" and got 3.860? At that point I stopped
cutting. My traps were designed for 3.890 but I had no way to
measure them.

Why did trimming 6" lower my resonant freq 8 kHz? I'm guessing
but could my traps be closer to 3.86 than 3.89? I'm afraid to cut
more.

Here are the results I got:

Band 2:1 SWR range Bandwidth Center Best SWR
160 1.862-2.040 0.178 1.951.0 1.0:1
80 3.759-3.916 0.157 3.837.5 1.0:1

Ken KG0WX


Well, I don't know what you used to measure, but don't discount measurement
accuracy.

The results speak for themselves.
73
--
Steve N, K,9;d, c. i My email has no u's.