RadioBanter - antenna lengths

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- - antenna lengths (https://www.radiobanter.com/antenna/80626-antenna-lengths.html)

There are some commonly occurring lengths for multi-band dipoles. I see
105', 135', and 140'. Is 140' preferred over 135'? If so, what is the
benefit of going from 135' to 140'? Just curious.

RB wrote:
There are some commonly occurring lengths for multi-band dipoles. I see
105', 135', and 140'. Is 140' preferred over 135'? If so, what is the
benefit of going from 135' to 140'? Just curious.

That's about a 3.5% difference. You get just about that much difference
between insulated and uninsulated wire. That much difference in length
doesn't much matter for non-resonant length dipoles. However, for
resonant dipoles, length certainly matters and 140 ft. would probably
be resonant outside of the 80m band. For instance, a resonant 1/2WL
dipole for 3.8 MHz would be about 123 foot long.
--
73, Cecil http://www.qsl.net/w5dxp

Hi RB, With multi-band dipoles you will have to use an antenna tuner.
The lengths you see in the charts are chosen such that the
antenna+feedline will have an impedance (hopefully) on multiple bands
that your antenna tuner can handle. Usually if a 135' dipole won't
load on a certain band, the feedline length is changed, because it is
easier than bringing down the dipole and changing to 140'.
Gary N4AST

On Tue, 25 Oct 2005 21:40:09 GMT, Cecil Moore wrote:

That's about a 3.5% difference. You get just about that much difference
between insulated and uninsulated wire. That much difference in length

Cecil, do you have theoretical or experimental support for the
inference that insulated vs uninsulated wire will be different in
length by around 3.5%? What thickness wire / insulation?

Owen
--

do you have theoretical or experimental support for the
inference that insulated vs uninsulated wire will be different in
length by around 3.5%? What thickness wire / insulation?

Owen

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

To calculate change in velocity due to plastic PVC insulation we need
to know :-

Wire diameter = d
Diameter over insulation = D
Height of wire above ground = H
Permittivity of insulating material = K

First calculate capacitance of bare wire to ground.
Then calculate capacitance of insulated wire to ground.
Velocity Factor = SquareRoot of their ratio.
Neither Terman's nor Kraus' Bibles will mention the following
formulae. So you'll just have to take my word for it.

Velocity Factor = SquareRoot( ( A + B ) / C )

Where -

A = K * Ln( 4 * H / D )

B = Ln( D / d )

C = K * Ln( 4 * H / d )

Example :
Bare wire diameter = 1.6 mm = 14 awg.
Diameter over insulation = 2.6 mm.
Height above ground = 10 metres = 33 feet.
Permittivity of insulation = 3.5

Velocity factor = 0.983
Or a decrease in resonant frequency of 1.7 percent which can nearly
always be forgotten about.

The electrical effect of the enamel on magnet wire is entirely
negligible. But it does protect the wire from atmospheric pollution.
----
Reg.

"RB" wrote in message
...
There are some commonly occurring lengths for multi-band dipoles. I see
105', 135', and 140'. Is 140' preferred over 135'? If so, what is the
benefit of going from 135' to 140'? Just curious.

Read the following:
http://members.dslextreme.com/users/w6wqc/antenna.html
The data on antenna efficiency is a misinterpretation of the NEC output
file. It refers only to copper losses, and does not include ground losses.
The discussion that "The length of an antenna wire is irrelevant" is the key
point made.

73,

Frank

There are some commonly occurring lengths for multi-band dipoles. I see
105', 135', and 140'. Is 140' preferred over 135'? If so, what is the
benefit of going from 135' to 140'? Just curious.

Pse look at the web: http://www.qsl.net/aa1ll/cfwire.htm
Depending on your location (length of the feeder and free space for the
dipole) consider Region 1, 3 or G5RV Region.

73 Chris SP7ICE

Owen Duffy wrote:
Cecil, do you have theoretical or experimental support for the
inference that insulated vs uninsulated wire will be different in
length by around 3.5%? What thickness wire / insulation?

Owen, that info came from Roy, W7EL, who built it into the latest
version of EZNEC. It has to do with the VF of insulated wire Vs
uninsulated wire which affects wire antennas as well as transmission
lines.

I discovered it for myself when I was building vertical loop
antennas many years ago. The equation 1005/f wasn't even close
using insulated wire.

For instance, my 130 foot bare copper wire dipole is resonant at
3.633 MHz according to EZNEC. Adding 0.05 inch of PE insulation
drops the resonant frequency to 3.526 MHz, about a 3% drop
according to EZNEC.
--
73, Cecil http://www.qsl.net/w5dxp

On Wed, 26 Oct 2005 00:15:11 +0000 (UTC), "Reg Edwards"
wrote:

do you have theoretical or experimental support for the
inference that insulated vs uninsulated wire will be different in
length by around 3.5%? What thickness wire / insulation?

Owen

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

To calculate change in velocity due to plastic PVC insulation we need
to know :-

Wire diameter = d
Diameter over insulation = D
Height of wire above ground = H
Permittivity of insulating material = K

First calculate capacitance of bare wire to ground.
Then calculate capacitance of insulated wire to ground.
Velocity Factor = SquareRoot of their ratio.
Neither Terman's nor Kraus' Bibles will mention the following
formulae. So you'll just have to take my word for it.

Velocity Factor = SquareRoot( ( A + B ) / C )

Where -

A = K * Ln( 4 * H / D )

B = Ln( D / d )

C = K * Ln( 4 * H / d )

Example :
Bare wire diameter = 1.6 mm = 14 awg.
Diameter over insulation = 2.6 mm.
Height above ground = 10 metres = 33 feet.
Permittivity of insulation = 3.5

Velocity factor = 0.983
Or a decrease in resonant frequency of 1.7 percent which can nearly
always be forgotten about.

The electrical effect of the enamel on magnet wire is entirely
negligible. But it does protect the wire from atmospheric pollution.

Thanks Reg. I had not doubt there was an effect, but it was the
arbitrary percentage figure that was implied irrespective of physical
parameters which could have a large range (such as your examples).

Clearly, the effect is a minor one in practical HF wire antenna cases
compared to the influence of nearby structures, ground conditions etc,
and one that is adequately dealt with by normal expectations of
trimming length of an antenna for resonance (where that is the
objective) rather than "designing" for the insulation factor.

Owen
--