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Old May 14th 04, 01:42 AM
Paul Logan
 
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Default TVI/ W3EDP question

Hi all,

ive been having a bit of a TVI problem with a neighbour whos receiving set
up isnt up to scratch ( Receives me great though and seems to transmit line
timebase sound very nicely too)

Anyhow I tried an experiment using an end fed wire the W3EDP - 85 feet of
wire tuned against a 17 feet counterpoise and said neighbours tv went nuts.
I was feeding the wire into the shack and using a tuner at the window frame
with the counterpoise dropped out the window from the second floor- dangling
with no connection to anything. Now on taking away the counterpoise and
adding a much longer counterpoise which runs underground the interference
problem cleared up.

Can someone explain to me exactly what was going wrong- was the counterpoise
radiating like mad with no path to ground?.

Also can someone suggest some simple antennas which work well against TVI?.

--
Paul Logan, MI3LDO,
Lisnaskea, N. Ireland.
Location: 54 - 15 N, 7 - 27 W in IO64GG

Low VHF & FM DX Page:
www.geocities.com/yogi540



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Old May 14th 04, 02:26 AM
Dave Platt
 
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Default

Hi all,

ive been having a bit of a TVI problem with a neighbour whos receiving set
up isnt up to scratch ( Receives me great though and seems to transmit line
timebase sound very nicely too)

Anyhow I tried an experiment using an end fed wire the W3EDP - 85 feet of
wire tuned against a 17 feet counterpoise and said neighbours tv went nuts.
I was feeding the wire into the shack and using a tuner at the window frame
with the counterpoise dropped out the window from the second floor- dangling
with no connection to anything. Now on taking away the counterpoise and
adding a much longer counterpoise which runs underground the interference
problem cleared up.

Can someone explain to me exactly what was going wrong- was the counterpoise
radiating like mad with no path to ground?.


The neighbor's TV might have been picking up direct radiation from the
counterpoise (yes, the counterpoise is going to radiate - it's half of
the antenna!). It's possible that the counterpoise was dangling close
enough to some household metal (e.g. a gutter, or the chickenwire in a
stucco wall) to be coupling a lot of RF into the building structure.

It's also possible that with this counterpoise you had a significant
antenna imbalance - enough to be creating an "RF in the shack"
problem, with RF appearing on your rig's chassis and coupling into
your house wiring, and getting into the TV set via the power mains.

Also can someone suggest some simple antennas which work well against TVI?.


In general, what you want to do is minimize the amount of RF which
gets into the receiving set via any path:

- via the antenna,
- via the power lines,
- via direct pickup to the chassis

This will reduce the problem of "fundamental overload", where the high
RF voltage levels from your transmitter force the RF circuitry in the
TV into nonlinear operation. TV sets _should_ be designed to deal
gracefully with strong RF signals outside of their operational
band(s), but they often are not.

One good way to minimize RF pickup by the TV is to simply increase the
distance between your own antenna, and the TV and its antenna and
power wiring. Also, use a balanced antenna (e.g. a dipole), with a
good balun at the antenna feedpoint - this will minimize the extent to
which your feedline radiates RF.

I've seen a claim that the Cobbwebb antenna (a dipole bent into a
near-circular shape, with the tips of the wires only a foot or so
apart) tends to cause less TVI than some other antennas... allegedly
because having the two high-voltage ends fairly close to one another
causes the resulting RF voltage fields to cancel out fairly rapidly as
one moves away from the antenna. I haven't experimented enough with
this sort of antenna configuration to know if it's true.

Other things which can help include:

- Install a high-pass filter right at the TV's antenna terminals.
This will block out the lower-frequency RF from your transmitter,
and help minimize fundamental overload in the TV's tuner.

- Add a ferrite RF choke to the TV's power cord, as close to the TV
chassis as possible. This will help minimize RF pickup via the
power mains and the TV's power cord. If the TV has wires to hook
it to external loudspeakers, or to other A/V components, clamp some
ferrites onto these wires as well.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!


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