On Wed, 10 Dec 2003 13:17:55 -0600, 'Doc wrote:



Yuri,
The reason I can't agree with the 5% thing is that I haven't
measured it, not that the number is correct or not. There is
a difference in length when using insulated/noninsulated wire,
I just can't say exactly what that difference is.

Since the environment in which the antenna is mounted/hung influences
the resonant frequency, I've never been able to see any difference
between insulated and bare copper wire.

As the things have never come out according to the formula, "for me"
I just "cut long and trim to fit".

I put up two 75 meter half wave sloping dipoles. They were cut
identical and even used the same make and model of balun. The coax
cable was the same length (LMR-400). Both were mounted at the same
height with the ends the same distance from the tower and ran at the
same angle. One ran to the NE to within about 30 feet of some trees.
The other ran to the south west over an open yard. Both worked well,
but there was between a 50 to 75 KHz difference in the resonant
frequency. The one over the open yard being the higher.

I didn't prune them as I use a line tuner to get the full band
coverage any way.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair?)
www.rogerhalstead.com
Return address modified due to dumb virus checkers

Since any calculation is only going to be approximate
(depending
on how/where the antenna is mounted) trimming is almost always
going
to be required. Since that 'tweaking' would take care of the
difference in insulated/noninsulated wire, I just haven't
bothered
about figuring it. If you have, then good, I don't have a
problem
with that...
'Doc

Since the environment in which the antenna is mounted/hung influences
the resonant frequency, I've never been able to see any difference
between insulated and bare copper wire.

As the things have never come out according to the formula, "for me"
I just "cut long and trim to fit".


Nothing wrong with that, trimming and fine tuning to accomodate the
surroundings.

I put up two 75 meter half wave sloping dipoles. They were cut
identical and even used the same make and model of balun. The coax
cable was the same length (LMR-400). Both were mounted at the same
height with the ends the same distance from the tower and ran at the
same angle. One ran to the NE to within about 30 feet of some trees.
The other ran to the south west over an open yard. Both worked well,
but there was between a 50 to 75 KHz difference in the resonant
frequency. The one over the open yard being the higher.


This is 1.3% difference, which I would attribute to capacitance from the trees,
ground variation or something in the vicinity. I would guess that the one
closer to the trees was lower in frequency. I tune my 160 m mobile L loaded
antenna by changing the loading wire slope or distance of the end from the
hood.

I didn't prune them as I use a line tuner to get the full band
coverage any way.


That is fine. The only problem with "plastic" shortening is when you are trying
to build say quad multielement antennas and those 5% can be really annoying. It
is better to know about it and compensate before taking the knife to operate on
the wires. Regular EZNEC can not accomodate "plastic" wires and it is
dissapointing to go through the effort of putting the monster up, only to find
it resonates in the CB band instead of middle of 10m.

Roger Halstead (K8RI & ARRL life member)

Yuri, K3BU

Roger Halstead wrote:

-------------
As the things have never come out according to the formula, "for me"
I just "cut long and trim to fit".

-------------

Hmm. Bet that 5% probably had something to do with that,
huh? And
the "check -n- chop" tuning method is probably the most common.
Works
too.
I would also suspect that the difference between
insulated/noninsulated
lengths is frequency sensitive. I've never had an 80 meter
antenna be 'off'
by something on the order of 5-6 feet, unless It was my fault
when doing the
measuring. I have had antennas for higher bands be 'off' by
something like
5%, though (even allowing for my 'elastic' yardstick)...
'Doc

To provide some idea of the magnitude of what the waffling is about -

A thick layer of insulation over an antenna wire is just
uniformly-distributed capacitance loading. As with ALL transmission lines,
an increase in capacitance per unit length results in a reduction in the
propagation velocity and a reduction in the resonant length.


Capacitance increase depends on wire diameter, diameter over insulation, and
permittivity of the insulating material. The velocity factor due to
insulation thickness in the case of a long, straight, antenna wire, can be
estimated as follows.


d = wire diameter.
D = diameter over insulation.
h = height above ground.
K = permittivity of insulation.

K can vary from 2.5 to 6 depending on material.


Calculate A = K*Ln( 4*h/d )
Calculate B = K*Ln( 4*h/D )
Calculate C = Ln( D/d )

Then VF = Sqrt( A / ( B + C ))

Decrease in antenna length due to insulation = ( 1 - VF )*100 percent.

And unless the insulation thickness is like 1" diameter coax with the braid
stripped off, for ordinary HF wire antennas at ordinary heights the pruning
will be lost in all the other things which might need a teeny bit of
pruning. As Roger advises "cut long and trim to fit". In practice, if a
tuner is in circuit, after taking the obligatory end-effect into account,
then I would guess pruning is seldom needed on simple wire antennas.
----
Reg, G4FGQ

The guns are silent.

"All quiet on the Western Front."

Isn't it amazing that a lil' drop of 'rithmetic has a greater smoothing
effect than "pouring OIL on troubled waters" ?

==========================

Reg wrote -

A thick layer of insulation over an antenna wire is just
uniformly-distributed capacitance loading. As with ALL transmission lines,
an increase in capacitance per unit length results in a reduction in the
propagation velocity and a consequent reduction in the resonant length.


Capacitance increase depends on wire diameter, diameter over insulation, and
permittivity of the insulating material. The velocity factor VF due to
insulation thickness in the case of a long, straight, antenna wire, can be
estimated as follows.


d = wire diameter.
D = diameter over insulation.
h = height above ground.
K = permittivity of insulation.

K can vary from 2.5 to 6 depending on material.


Calculate A = K*Ln( 4*h/d )
Calculate B = K*Ln( 4*h/D )
Calculate C = Ln( D/d )

Then VF = Sqrt( A / ( B + C ))

Decrease in antenna length due to insulation = ( 1 - VF )*100 percent.


And unless the insulation thickness looks like 1" diameter coax with the
braid stripped off, for ordinary HF wire antennas at ordinary heights the
pruning will be lost in all the other things which might need a teeny bit of
pruning. As Roger advises "cut long and trim to fit". In practice, if a
tuner is in circuit, after taking the obligatory end-effect into account,
then I would guess pruning is seldom needed on simple wire antennas at
ordinary heights.
----
Reg, G4FGQ