I wouldn't figure
the 5% in the equation, you can always trim the extra off.

Steve
kb8viv


You can do it hard way, or the easy way. If you know approximate shortening
factor, it will save you monkeying with the wire cutters and you could get
right on the first try, which is more important when you building multielement
wire antennas like quads.

If you like exercise with pliers, then by all means, take the spool of wire and
start cutting :-)

73 Yuri

Yuri:

You are answering a question that was not asked. Your answer is excellent if
the question we Is it necessary, when calculating element length, to
consider the type of wire used?

If you calculate based on insulated wire and then build it out of bare wire,
it ain't going to work well either.

Assuming one calculates for whatever radiating thingy one is using, it makes
little difference whether that thingy is bare or insulated.

Paul AB0SI

The question was:
"What are pros and cons of using insulated twisted pair wire for wire
antenna projects? (its what I have handy) Comments invited."

Lacking more specific question (didn't say white kind of "wire antenna
projects" resonant or random length), I pointed out "cons" - the effect of
insulation on calculation and modeling the antenna's physical length vs.
electrical. 5% shortening from insulation makes difference if you end up in 10m
ham band or 11m CB band, that is 1.4 MHz difference and that IS significant.

Inquirer can decide for himself if it is important for his case (making
resonant antennas) or not (non resonant, long wire antennas).

If I offended anybody's insulation, then I am sorry :-)

Yuri, K3BU

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 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

One who Art calls Doctor writes:

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.


I described what it is exactly as I measured it using #12 wire on a 10m 3
element quad. Calculated it with (cheap) EZNEC, I put it up, it ended up smack
in CB band. Calculated the offset, for the red insulation I was using (helps to
emit red hot electrons :-) it was 5%. Adjusted the length and bingo, right
where I wanted it.

Expensive EZNEC using NEC4 can include insulation in calculations and provides
more accurate results and it can be used to show the exact, more-less
difference due to plastic coated wires (according to Roy d'Eznec).

So if you didn't measure it yet, everyone is going to call me a liar? :-)
Maybe I should start taking bets? Haven't you guys learned to put a little
trust in my 45 years of haming, beating contest records and generaly honest and
being mostly right?


Yuri

Yuri, the one 'Artfully' called "Tailgator",
You were on a roll and doing fine till you said 'contester'.
That's when you blew your credibility. Dang!
'Doc

PS - Are you 'Tailgator' #1 or #2? I have no idea of 'our'
ranking.

Yuri, the one 'Artfully' called "Tailgator",
You were on a roll and doing fine till you said 'contester'.
That's when you blew your credibility. Dang!
'Doc

PS - Are you 'Tailgator' #1 or #2? I have no idea of 'our'
ranking.

That I dunno, this is why I operate contests so I can find my number. There is
no BS or peesychology there.
I still like to contest and my credit is good, I get lot of credit card offers.

Yuri, BUm BUm

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