I'm a noob, and I'm considering installing a wire antenna using the
sole tree on my lot as one end. My question is "Is it dangerous if
the antenna touches the tree branches/leaves during transmit?" I'm
less worried about performance right now and more safety.

Marc
W5AMP

marc wrote:
I'm a noob, and I'm considering installing a wire antenna using the
sole tree on my lot as one end. My question is "Is it dangerous if
the antenna touches the tree branches/leaves during transmit?" I'm
less worried about performance right now and more safety.

Marc
W5AMP

how much power?

I used to have a dipole outside my apartment and the wires would touch
some of the leaves/branches. I used a tuner at the feedpoint. On 40m
(only), running 100W, it would arc to the tree on damp nights.

So, the real answer is, it depends. 100W dissipated in one spot could
probably start a fire. a few watts lost in miscellaneous crackling and
sparks, probably not.

If your wire antenna goes to an insulator, then to a rope/cable that
goes to the tree, then you're probably much better off than just
flinging the antenna wire through the branches. (the latter is popular
with backpack portable...)



And, if you're that close to the tree, you're probably inducing some RF
currents in the tree, so RF fields safety is an issue. Again, it
depends. Power, operating mode, style of operation, whether there's any
access controls to the area where the tree is, how high the antenna is,
how big the tree is, etc.

"Jim Lux" wrote in message
...
marc wrote:
I'm a noob, and I'm considering installing a wire antenna using the
sole tree on my lot as one end. My question is "Is it dangerous if
the antenna touches the tree branches/leaves during transmit?" I'm
less worried about performance right now and more safety.

Marc
W5AMP

Marc - I have a thickly wooded side yard where I run 80 and 40 meter dipoles
through and over a number of tree limbs. I don't believe that at HF
frequencies (VHF & UHF is different) that the limbs and leaves make any
significant difference - unless the RF power is actually shorting into the
limbs and leaves. It will do this, if the wires are not insulated.

I run a KW (when needed) to dipoles that use #14 stranded house wire. The
antenna wires are threaded through the limbs and leaves, resting on some of
the branches. The dipoles work quite well. I do not see any fluxuation in
the SWR when it rains or the wind is blowing, so I believe the nominal
600-volt wire insulation is sufficient to prevent shorting. (I try to keep
the ends of the wires off the limbs.) The insulation on the wire will not
hurt your radiated radio signal. Good luck. Andy K4YKZ (KN4YKZ in '58)

You can match any thing to a transmitter with a good antenna tuner, the
question is the radiation resistance low enough to efficently transfer
energy to free space?

When using a G5RV that was close but not touching the branch of tree I got a
flat match, could receive well but did not get out worth a damn. Moving the
antenna to an area where there were no trees and using 2 30 Rohn telescopic
masts as end poles made a big difference, got a flat match, RX and TX were
great.

My 2 cents worth on the subject but I as usual someone will say they
transmit great with the branchs laying right on the wire.

I agree with the arcing and fire saw that happen when a wet branch connected
with a 660 VAC line on a utility pole once.

Homer J

"marc" wrote in message
...
I'm a noob, and I'm considering installing a wire antenna using the
sole tree on my lot as one end. My question is "Is it dangerous if
the antenna touches the tree branches/leaves during transmit?" I'm
less worried about performance right now and more safety.

Marc
W5AMP

Homer J wrote:
You can match any thing to a transmitter with a good antenna tuner, the
question is the radiation resistance low enough to efficently transfer
energy to free space?

The radiation resistance is simply an imaginary circuit element whose
dissipation represents the energy transferred to free space. So 100% of
the energy supplied to the radiation resistance, whatever its value, is
transferred to free space -- by definition.

That said, the radiation resistance does play a role in determining what
percentage of the power supplied to the antenna is radiated and what
percentage is dissipated as heat. The formula is:

Efficiency = Rrad / (Rloss + Rrad)

where Rrad is the radiation resistance and Rloss the loss resistance.
It's essential that both values be referred to the same point. So you
can see from this simple formula that for a fixed amount of loss, the
higher the radiation resistance, the higher the efficiency. But also
keep in mind that the signal from a 79% efficient antenna is only 1 dB
lower than from a 100% efficient antenna, so there's not much point in
worrying about the last few (or, often, many) percent.

The feedpoint impedance, part of which is the radiation resistance, has
to be matched in order to get power to it in the first place. And most
tuners are efficient only over a limited range of impedances. So very
high or low radiation resistance can result in loss in the tuner,
reducing your overall efficiency.

When using a G5RV that was close but not touching the branch of tree I
got a flat match, could receive well but did not get out worth a damn.
Moving the antenna to an area where there were no trees and using 2 30
Rohn telescopic masts as end poles made a big difference, got a flat
match, RX and TX were great.

A good impedance match doesn't tell you anything about the antenna or
system efficiency. But on HF if you can hear signals a lot better than
they can hear you, it's a sign of poor system (antenna and tuner)
efficiency.

My 2 cents worth on the subject but I as usual someone will say they
transmit great with the branchs laying right on the wire.

The effect of branches touching the wire depends very strongly on where
along the antenna the branches touch relative to the points of highest E
field. So two people could have very different results from touching
branches. Also, you could have very different results on different bands
from the same antenna and same branches.

. . .

Roy Lewallen, W7EL