I like the 75 ohm because it is a good match to the dipole. If you do
the 300 ohm line and make the driven element a folded dipole out of
the same twin lead, it will also be a good match.

Usually I have found it works well even without a good match on a
dipole because the twin lead is generally a low loss line and the
radiation from the feed line does not really reduce the usability of
the antenna.

A poor match will be inclined to generate TVI and sometime get RF back
into the shack. Also, some of the newer radios will not drive anything
other than 50 ohms.

Most likely you have a tuner or you would not be considering a
balanced line anyway.

If you go with the 75 ohm scheme and feed it directly from a "modern"
transceiver it will probably still work pretty well but it will be
likely to generate TVI.

If luck is on your side, the 300 ohm feed MIGHT work too, but not as
you intended.

Whatever you choose, start at low power & work your way up.

I cannot figure out what you said that generated so many juvenile
responses. It sounds like a reasonable question to me.

On Tue, 26 Jul 2005 15:40:38 -0400, Eric Delongchamp
wrote:





Hi !



I want to put up a 40m dipole antenna and my question is what a
transmission line to use, I have two defferent type of twin lead to
choose from, one is 75 ohm and a TV 300 ohm line. I'm using Yeasu 747
GX and MFJ 949 E for equiment. Thanks



Eric D.

John Ferrell wrote:
If luck is on your side, the 300 ohm feed MIGHT work too, but not as
you intended.

Ever noticed that luck favors the knowledgeable?

Multiples of 1/2WL length will work for any characteristic
impedance. The 300 ohm twinlead is less lossy than 72 ohm
twinlead. If 64 ft. is a convenient length for his feedline,
300 ohm twinlead is a good choice for a 1/2WL dipole probably
not requiring a tuner. An added advantage is that it may also
work well with a tuner on some of the higher frequencies.
--
73, Cecil http://www.qsl.net/w5dxp

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"John Ferrell" wrote:
A poor match will be inclined to generate TVI...
_________________

Could somebody please explain why feedline SWR should produce any more
general TVI in the neighborhood than if the match is perfect?

If the match is perfect, the antenna radiates all the power from the tx
(less line loss). If anything else radiates part of that tx power (the
feedline, you say), the total radiated power still can be no more than when
the antenna radiates all of it. So if tx harmonic levels are the same in
both cases, why should "feedline radiation" have much affect on TV receivers
in the neighborhood?

Are you talking about some very localized effects (tx feedline running next
to your own TV set or TV antenna, etc)?

RF

Richard Fry wrote:
"John Ferrell" wrote:

A poor match will be inclined to generate TVI...

_________________

Could somebody please explain why feedline SWR should produce any more
general TVI in the neighborhood than if the match is perfect?

If the match is perfect, the antenna radiates all the power from the tx
(less line loss). If anything else radiates part of that tx power (the
feedline, you say), the total radiated power still can be no more than
when the antenna radiates all of it. So if tx harmonic levels are the
same in both cases, why should "feedline radiation" have much affect on
TV receivers in the neighborhood?

Are you talking about some very localized effects (tx feedline running
next to your own TV set or TV antenna, etc)?

RF

It sounds like you've confused feedline SWR with feedline radiation.
Those are two different effects. A line can have 1:1 SWR and radiate, or
10:1 and not radiate. SWR is determined solely by the differential load
impedance and the line Z0. Feedline radiation is determined only by the
amount and distribution of common mode current on the transmission line
and of course its length.

The line SWR can be altered only by impedance matching at the load or
changing the line's Z0. Feedline radiation is reduced by using one or
more current baluns (common mode chokes) or a tuner with an inherently
balanced output such as link coupling. While changing the feedline
radiation can change the SWR (by changing the radiating part of the
antenna), changing the SWR by altering the differential load impedance
has no effect on the common mode current and hence the line radiation.
(Anything which alters the common mode impedance of the antenna, which
depending on its design a differential matching circuit might do, can
change the line radiation. But it's not because of the change in SWR;
it's because of how the matching circuit alters the common mode impedance.)

Only feedline radiation is associated with TVI; SWR isn't.

Feedline radiation can cause increased TVI and other RFI if the
transmission line is routed in a way that it can couple to power lines,
telephone lines, TV cable, and other conductors which take RF into homes
and close to TV sets and other electronic devices. (I've had more
trouble with modern telephones than anything else.) Radiating antennas
can, of course, do the same thing and to an even greater degree -- an
attic transmitting antenna can often cause problems, for example. But
feedline radiation can increase the RFI potential of an antenna that's
otherwise far away from other conductors. The problem is simply one of
effectively bringing part of the antenna into the house. It is, as you
say, a localized effect, although it can affect a neighborhood by
conduction and radiation from the conductors into which the feedline
radiation is coupling.

Roy Lewallen, W7EL

Roy:

Even if you have me bit-bucketed, thanks for that info.

I am one who was ignorant to what you present in your text--I always expected
increased line radiation with a mismatch. Now that you have stated it in text,
it is plain a balun would prevent most if not all of this...

I will re-evaluate and brush up on what I have certainly missed before.

Warmest regards,
John

"Roy Lewallen" wrote in message
...
Richard Fry wrote:
"John Ferrell" wrote:

A poor match will be inclined to generate TVI...

_________________

Could somebody please explain why feedline SWR should produce any more
general TVI in the neighborhood than if the match is perfect?

If the match is perfect, the antenna radiates all the power from the tx
(less line loss). If anything else radiates part of that tx power (the
feedline, you say), the total radiated power still can be no more than when
the antenna radiates all of it. So if tx harmonic levels are the same in
both cases, why should "feedline radiation" have much affect on TV receivers
in the neighborhood?

Are you talking about some very localized effects (tx feedline running next
to your own TV set or TV antenna, etc)?

RF

It sounds like you've confused feedline SWR with feedline radiation. Those
are two different effects. A line can have 1:1 SWR and radiate, or 10:1 and
not radiate. SWR is determined solely by the differential load impedance and
the line Z0. Feedline radiation is determined only by the amount and
distribution of common mode current on the transmission line and of course
its length.

The line SWR can be altered only by impedance matching at the load or
changing the line's Z0. Feedline radiation is reduced by using one or more
current baluns (common mode chokes) or a tuner with an inherently balanced
output such as link coupling. While changing the feedline radiation can
change the SWR (by changing the radiating part of the antenna), changing the
SWR by altering the differential load impedance has no effect on the common
mode current and hence the line radiation. (Anything which alters the common
mode impedance of the antenna, which depending on its design a differential
matching circuit might do, can change the line radiation. But it's not
because of the change in SWR; it's because of how the matching circuit alters
the common mode impedance.)

Only feedline radiation is associated with TVI; SWR isn't.

Feedline radiation can cause increased TVI and other RFI if the transmission
line is routed in a way that it can couple to power lines, telephone lines,
TV cable, and other conductors which take RF into homes and close to TV sets
and other electronic devices. (I've had more trouble with modern telephones
than anything else.) Radiating antennas can, of course, do the same thing and
to an even greater degree -- an attic transmitting antenna can often cause
problems, for example. But feedline radiation can increase the RFI potential
of an antenna that's otherwise far away from other conductors. The problem is
simply one of effectively bringing part of the antenna into the house. It is,
as you say, a localized effect, although it can affect a neighborhood by
conduction and radiation from the conductors into which the feedline
radiation is coupling.

Roy Lewallen, W7EL

John, this business about problems caused by radiation from feedlines
is very much exaggerated. It occurs far less frequently than it is
mentioned on this newsgroups. It's just something to waffle about.

It's perfectly obvious far more near-field interference occurs due to
radiation from the ANTENNA. Especially when the beam is pointing
towards the house.

The antenna is designed and intended to radiate. Whereas the feedline
only does it because of occasional imperfections and by accident.
Most of the time it is not detectable as interference from either
antenna or the line.

{Actually, it is nonsense to discuss things in terms of separate
radiation from line and antenna. I am obliged to do so here to make
myself understood and avoid accusations of trolling.}
----
Reg

In article , Richard Fry wrote:

Could somebody please explain why feedline SWR should produce any more
general TVI in the neighborhood than if the match is perfect?

My guess is that this belief is true in some cases... specifically the
following:

- You have a dipole, or other balanced antenna
- You're feeding it with coax
- The dipole has a feedpoint impedance which is high (resistive
and/or reactive) at the frequency of interest
- You don't have a balun at the feedpoint, or your balun has only a
modest amount of choking reactance
- Your coax is somewhere near a half-wavelength long (or a multiple
thereof) from the antenna to the point at which its first ground
occurs.

In this situation, you're likely to have a lot of RF on the outside of
the coax, as its braid will present a path for the ground-side current
which is significantly less than that of the antenna. You could end
up with "RF in the shack" problems, or the various other problems
which can result from having high levels of RF flowing near other
consumer-electronics devices.

If your feedline happens to be parallel to a cable-TV line, its
radiation may tend to induce a significant RF current on the outside
of the cable-TV coax, and might bleed to the inside. This could
result in fundamental-overload problems in the TV or distribution
amplifier.

If your feedline runs near a metal gutter or downspout, or near a
stucco wall with chicken-wire mesh in it, it might induce strong
enough RF currents to cause rectification noise in the joints, and the
generation of undesired harmonics of your transmit frequency which
might fall into the TV frequency band. Similar generation of
harmonics can occur, I gather, if you push a ferrite-core balun too
hard, and saturate the ferrite.

If I recall correctly, 6-meter operation is notorious for causing RFI
due to both fundamental overload (Channel 2 is often affected due to
its proximity to the 6-meter band) and to second-harmonic spurs
falling in the FM band.

If you've got RF coming back down into the shack, you might have some
of the RF-ground current getting into your household AC power ground,
and bleeding into the TV sets via their line cords.

All of these are things which can occur with any antenna, of course...
they're just exaggerated (to whatever extent) by RF-on-the-coax due to
the RF's greater proximity to the affected TVs.

In all of these cases, I'd expect problems to be more apparent for
nearby TV sets (same building) than those far away. In urban areas,
with people living in apartments or condos, the "neighborhood" can
consist of quite a few people living in individual households within
100 feet of the transmitter and antenna.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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