On Sun, 25 Dec 2005 04:05:07 GMT,
wrote:

On Sun, 25 Dec 2005 01:51:01 -0000, (Dave Platt)
wrote:

I leave only this.

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.

True, but that's not the situation we're dealing with here.

If you place a shorted quarter-wave section directly at the
transmitter's terminals, in parallel with the antenna feedline, then
the transmitter "sees" the impedance of the feedline (its usual load)
in parallel with the impedance of the shorted stub (very high). The
net impedance is that of the load (the admittance of the shorted stub
is nearly zero) and the transmitter does not "notice" the presence of
the shorted stub.

That's not the situation which occurs if the feedline itself is
shorted 1/4 waveline towards the load

Only if the short is perfect or the frequency is low enough.
Even for shorted stubs doing it right requires a bit of effort
as even a loop at the end causes interesting effects.

In that situation, the *only* thing that the transmitter will see is
the shorted quarter-wavelength "stub" between itself and the short.
The impedance at the point of the short is nearly zero - it's the

Nearly zero is not zero and it's not a constant with frequency.
At 3.5mhz I will agree with you. At 350mhz it's going to show
something different. At 3500mhz, who knows?

impedance of the short itself, in parallel with the impedance of the
antenna as seen when looking up the remainder of the feedline. No

However is it parallel or is it some complex reflection of the length
to the antenna.

matter what the antenna's impedance is, the very low impedance of the
short itself is going to dominate the parallel combination. The
resulting near-zero-ohm combination will be transformed, by the
quarter-wavelength distance back to the transmitter, so that it
appears as an open circuit to the transmitter.

The transmitter cannot, in effect, "see past the short circuit" to the
antenna itself.

The same is true no matter how far up the feedline from the
transmitter the short/pin happens to be. At the point of the short,
the impedance is going to be nearly zero, and this near-zero impedance
will be transformed to some other value on the same very-high-SWR
circle (neglecting consideration of feedline loss, of course).

No matter where you pin the coax, the transmitter is going to be
unhappy.

If a short appeared near a 1/4 wave node at operating frequency
it might go unnoticed.

Different situation, I'm afraid.

If you have an antenna analyzer, try it out for yourself. Take an
arbitrary-length section of RG58 with a 50-ohm load at one end and a
BNC at the other. Run it into a BNC "T". Out the other leg of the T,
run an adjustable length of RG-58 to the antenna analyzer. You ought
to measure 50 ohms in this situation.

Now, stick a short directly across the third branch of the T connector
("pinning" the coax, so to speak), and see what your analyzer tells
you. It may read high-Z, or low-Z, or intermediate-Z with a lot of
reactance... but it'll be a high indicated SWR, and it won't be
anywhere near 50+j0.

Will it be greater tha say 500+janything? Depends on frequency.

Actually using a tee and doing that creates a working stub that is
tuned in the low microwave region and something reactive below
that. In fact the open Tee at high uhf is also a trouble maker.

Then, disconnect the antenna from the "T". The impedance and
indicated SWR won't change significantly.

But t does, depending on frequency and line length on the other side
of the short. Try it for an exact 1/2wave from source to load and pin
at 1/4wave point. the reflected impedences at both ends will be
in play. The pin is only a perfect short at very low frequencies
as you go up the "short" gets "longer" and behaves as a reactance.
It's a complex circuit and there are lumped analogs.

Try changing the length of the RG58 between the "T" and the analyzer.
You'll get a different Z value with the short in place (whether the
antenna is or is not attached) but it'll still have a really high SWR,
no matter what coax length you choose.

Its easier to vary F than change length. But yes you will see varying
impedences (R+J) all over the map.

I keep saying a perfect short would behave as stated. Most real
world components like that pin would be a pain at 3.5mhz and
something else at 432mhz.

Allison
KB!gmx

Try it and you will be surprised.

73
Gary K4FMX

On Sun, 25 Dec 2005 20:22:05 GMT,
wrote:

I have at 2400mhz too! Down around DC (sub 30mhz) your "pin"
is a short and the effects are mostly (though measurable) a shorted
coax with all the effects as expected. As you get up there in
frequency the "short" as described doesn't behave as it did at DC.

The problem is similar to another thread concerning real world
components where the discussion finally recognized that like other
real world components a short is not always what it may look like.

Sigh...
I can't differentiate between you and that other chap on this issue.
You both cite perfectly legitimate grounds and come to entirely
separate conclusions. You can't both be right, but neither of you seem
to be wrong!
Can we focus down on *one* issue to avoid disappearing up our own
backsides: as far as the tx is concerned, is the portion of the feed
line beyond the pin relevant at all or does it effectively cease to
exist, as would be the case at VLF/DC?
--

"What is now proved was once only imagin'd" - William Blake

On Sun, 25 Dec 2005 12:44:12 -0500, Gary Schafer
wrote:


Try it and you will be surprised.

73
Gary K4FMX

I have at 2400mhz too! Down around DC (sub 30mhz) your "pin"
is a short and the effects are mostly (though measurable) a shorted
coax with all the effects as expected. As you get up there in
frequency the "short" as described doesn't behave as it did at DC.

The problem is similar to another thread concerning real world
components where the discussion finally recognized that like other
real world components a short is not always what it may look like.

Allison

In article ,
Paul Burridge k wrote:

I have at 2400mhz too! Down around DC (sub 30mhz) your "pin"
is a short and the effects are mostly (though measurable) a shorted
coax with all the effects as expected. As you get up there in
frequency the "short" as described doesn't behave as it did at DC.

The problem is similar to another thread concerning real world
components where the discussion finally recognized that like other
real world components a short is not always what it may look like.

Sigh...

I can't differentiate between you and that other chap on this issue.
You both cite perfectly legitimate grounds and come to entirely
separate conclusions. You can't both be right, but neither of you seem
to be wrong!

Can we focus down on *one* issue to avoid disappearing up our own
backsides: as far as the tx is concerned, is the portion of the feed
line beyond the pin relevant at all or does it effectively cease to
exist, as would be the case at VLF/DC?

It's really a question of the problem domain - that is, what are the
frequencies involved, and what are the sizes of the feedline and the
"width" of the shorting bar/pin? Allison and I have been talking
about rather different sets of test conditions. I think we're
actually in "violent agreement" about what actually goes on.

At upper-UHF and microwave frequencies, I agree that Allison is correct.
The length of the pin is a significant fraction of a wavelength, and
it thus does not behave as a true short circuit - rather, it's an
inductor of significant value shunted across the transmission line.
At these frequencies, in this problem domain, you have to consider the
shunt combination of two non-zero impedances. "Shorting" the feedline
with this 'straight pin' inductor will probably have a significant
effect on the impedance seen by the transmitter, but it won't be as
simple as I had portrayed.

At HF (and, I think, VHF up through the 2-meter frequency range) a
shorting pin of perhaps 1/4" in length is a negligible fraction of a
wavelength long. Whatever small amount of inductance it introduces
will have a reactive impedance whose magnitude is far below that of
the 50-ohm load, and its very high admittance will swamp the lower
admittance of the load. Hence, the load impedance will be of
negligible importance in deciding what the transmitter "sees" at these
frequencies, under these conditions... the transmitter "sees" only the
impedance of the short itself, transformed by however much line is
between transmitter and short.

If I can find a scrap BNC male connector, and make a shorting-plug out
of it, I'll run the coax-and-T experiment I suggested, and post some
actual numbers for the systems's behavior at those frequencies I can
coerce out of my MFJ-269.

--
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!

If I can find a scrap BNC male connector, and make a shorting-plug out
of it, I'll run the coax-and-T experiment I suggested, and post some
actual numbers for the systems's behavior at those frequencies I can
coerce out of my MFJ-269.

OK, I ran the test, and the results are pretty much as I had
anticipated. Although the test arrangement and gear isn't
lab-standard, I think it's good enough to confirm the basic principles.

Test equipment: MFJ-259 antenna analyzer, with an N-to-BNC adapter.

Transmission line and load: 12' length of RG-58 coax, with a 50-ohm
Ethernet terminator attached to the end.

Shorting insertion: an additional 6' length of RG-58, with a couple of
BNC "T" adapters, which can be inserted between the analyzer and the
T-line-and-load. If the shorting plug is inserted it'll be 6' from
the analyzer and 12' from the load.

Short circuit: a male BNC plug, with a short inserted between center
pin and shell as far down inside the shell as possible.

First test: check quality of T-line and load, using only the 12'
section of RG-58 between analyzer and load. Note that the Imag(load)
numbers do not indicate the sign of the reactance... the MFJ won't do
that, alas.

Frequency Real(load) Imag(load) Indicated SWR
2.5 52 4 1.0
5 58 6 1.2
10 57 7 1.2
15 49 1 1.0
20 56 6 1.1
50 54 2 1.1
144 51 4 1.0
166 59 8 1.2
440 1.4

Impression: either the RG-58 or the Ethernet terminator isn't
exactly 50 ohms (not unexpected) or the MFJ's calibration isn't
perfect (likewise) but the figures are good enough to let us draw some
reasonable conclusions.

Second test: insert the 6' RG-58 and its T connectors between the
analyzer and the 12' section. See how much this additional length of
line, and the parasitics of the T connectors, affect the load and SWR.

Frequency Real(load) Imag(load) Indicated SWR
2.5 54 4 1.1
5 59 5 1.2
10 50 10 1.2
15 49 1 1.0
20 51 8 1.1
50 48 5 1.1
144 50 4 1.0
166 40 2 1.2
440 1.1

Impression: the measured values change a bit, but the SWRs are close or
identical (save for the 440 measurement). The additional length of
line, and the parasitics from the T connectors, are shifting things
around a bit (especially at 440)... not unexpected.

Third test: connect the shorting plug to one of the T connectors. See
what "pinning the line" does to the analyzer's view of the load.

Frequency Real(load) Imag(load) Indicated SWR
2.5 0 6 31
5 0 12 31
10 1 26 31
15 1 43 31
20 2 70 31
50 0 44 28.7
144 7 33 15.4
166 166 353 13.0
440 5

Impression: yeah, that looks like a short circuit, transformed via a
feedline. Nothing close to a 50-ohm-resistive load shows up at any of
the test frequencies.

Fourth test: disconnect the 50-ohm load from the end of the 12' line,
creating an abrupt change in the load impedance. See what this does
to the figures from the third test - how much of the antenna load
change gets back "around" the short?

Frequency Real(load) Imag(load) Indicated SWR
2.5 0 6 31
5 0 12 31
10 1 25 31
15 1 43 31
20 2 70 31
50 6 44 29.0
144 6 30 18.5
166 133 317 14.1
440 5

Impression: changing the antenna load from 50-ohms-resistive to
near-infinite made a slight change in the impedance seen by the
analyzer, but not much at all at any frequency. The presence of the
short circuit 6 feet from the analuzer is still dominating the load
that the analyzer "sees".

Special-distance test: with the short, and the 50-ohm load both in
place, sweep the analyzer frequency around the ranges at which the
analyzer-to-short distance is around 1/4 and 1/2 wavelength. See if
we can "see past" the short under these conditions.

Measurements: with the short circuit in place, and a 50-ohm load at
the end of the 12' line, an impedance peak (Z1500 ohms) is noted when
sweeping between 32.14 MHz and 32.94 MHz. An impedance minimum
(Real(load) of 2-3 ohms, Imag(load) of 0 ohms) is noted between 65.7
MHz and 65.85 MHz. The indicated SWR remains high (22 or above) at
all frequencies between the impedance maximum and impedance minimum...
there's no point at which the load resembles anything like "50 ohms
resistive, little reactive component".

Removing the 50-ohm load, and thus open-circuiting the antenna
feedline, has no significant effect on the impedance as seen at the
quarter-wavelength maximum, at the half-wavelength minimum, or at
points in between.

Impression: we cannot "see past" the short circuit, no matter whether
it's a quarter-wavelength from the transmitter, a half-wavelength, or
some distance in between these two. The analyzer "sees" only what
would be expected for a short circuit, transformed by 6' of coax.

Short-circuit quality test: measure impedance of the shorted BNC plug.

At 16 MHz and below, the MFJ-269 shows it as 0+0j. Above 16 MHz, some
reactance shows up... 2 ohms at 28 MHz, 3 ohms at 42 MHz, 4 ohms and
55 MHz, 8 ohms at 112 MHz, 10 ohms at 144 MHz, 12 ohms at 169 MHz.
The inductance of the shorted plug itself, and/or the length of the
N-to-BNC adapter on the analyzer, is having some effect. SWR remains
unreadably high: 31 up through VHF and 5 on 440. It's not the
highest-quality "short circuit" in the world, for certain, but at HF
and VHF it's close enough for our purposes here.

So... what do I conclude from this?

I conclude that at HF and VHF frequencies, if you accidentally short
your transmitter-to-antenna coaxial feedline (with a pin, nail, or a
loose strand of coax braid which "gets loose" inside a connector),
you're going to present your transmitter with an unrealistic load
(high, low, or nastily reactive) and that the power flow up the
feedline is going to stop at the short circuit.

Since no "short circuit" in the real world is going to have 0+0j
impedance, this isn't *absolutely* true, but you can probably consider
it to be *practically* true and correct at these frequencies, with
this sort of shorting.

I further conclude that with respect to the above, there's no 'magic'
about shorts which happen to occur at a quarter-wavelength distance
from the transmitter. In this situation, the transmitter will 'see'
something which behaves like a quarter-wavelength shorted stub... the
load further up the transmission line at the antenna remains
'invisible' to the transmitter.

At high-UHF and SHF/microwave frequencies, where a direct "short" is
likely to be a significant fraction of a wavelength, then the
additional reactance of the "short" will certainly affect how the
undesired connection affects what the transmitter "sees". It may
hurt, or help (helping to "tune out" reactance from the antenna
itself), or may be completely invisible. Allison is entirely correct
on this point, under these particular conditions.

--
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!

On Sun, 25 Dec 2005 23:06:32 -0000, (Dave Platt)
wrote:

Second test: insert the 6' RG-58 and its T connectors between the
analyzer and the 12' section. See how much this additional length of
line, and the parasitics of the T connectors, affect the load and SWR.

Frequency Real(load) Imag(load) Indicated SWR
2.5 54 4 1.1
5 59 5 1.2
10 50 10 1.2
15 49 1 1.0
20 51 8 1.1
50 48 5 1.1
144 50 4 1.0
166 40 2 1.2
440 1.1

As expected.

Impression: the measured values change a bit, but the SWRs are close or
identical (save for the 440 measurement). The additional length of
line, and the parasitics from the T connectors, are shifting things
around a bit (especially at 440)... not unexpected.

Third test: connect the shorting plug to one of the T connectors. See
what "pinning the line" does to the analyzer's view of the load.

Frequency Real(load) Imag(load) Indicated SWR
2.5 0 6 31
5 0 12 31
10 1 26 31
15 1 43 31
20 2 70 31
50 0 44 28.7
144 7 33 15.4
166 166 353 13.0
440 5

Impression: yeah, that looks like a short circuit, transformed via a
feedline. Nothing close to a 50-ohm-resistive load shows up at any of
the test frequencies.

Key thing is somewhere bwtween 144 and 440mhz something is happening
that is not a short circuit but, not tested either. Also a SWR of 5
while high tells us that collective losses (RG58) and instrument
limitations are hiding information.

Fourth test: disconnect the 50-ohm load from the end of the 12' line,
creating an abrupt change in the load impedance. See what this does
to the figures from the third test - how much of the antenna load
change gets back "around" the short?

Frequency Real(load) Imag(load) Indicated SWR
2.5 0 6 31
5 0 12 31
10 1 25 31
15 1 43 31
20 2 70 31
50 6 44 29.0
144 6 30 18.5
166 133 317 14.1
440 5

Impression: changing the antenna load from 50-ohms-resistive to
near-infinite made a slight change in the impedance seen by the
analyzer, but not much at all at any frequency. The presence of the
short circuit 6 feet from the analuzer is still dominating the load
that the analyzer "sees".

Again at the spot frequencies tested. Lossy coax becomes a factor
at 200mhz (12ft at 440 is 1db loss).

Special-distance test: with the short, and the 50-ohm load both in
place, sweep the analyzer frequency around the ranges at which the
analyzer-to-short distance is around 1/4 and 1/2 wavelength. See if
we can "see past" the short under these conditions.

Measurements: with the short circuit in place, and a 50-ohm load at
the end of the 12' line, an impedance peak (Z1500 ohms) is noted when
sweeping between 32.14 MHz and 32.94 MHz. An impedance minimum
(Real(load) of 2-3 ohms, Imag(load) of 0 ohms) is noted between 65.7
MHz and 65.85 MHz. The indicated SWR remains high (22 or above) at
all frequencies between the impedance maximum and impedance minimum...
there's no point at which the load resembles anything like "50 ohms
resistive, little reactive component".

Try it again using shorter (by 1/4) cables at 4x the frequency.

Removing the 50-ohm load, and thus open-circuiting the antenna
feedline, has no significant effect on the impedance as seen at the
quarter-wavelength maximum, at the half-wavelength minimum, or at
points in between.

Impression: we cannot "see past" the short circuit, no matter whether
it's a quarter-wavelength from the transmitter, a half-wavelength, or
some distance in between these two. The analyzer "sees" only what
would be expected for a short circuit, transformed by 6' of coax.

Short-circuit quality test: measure impedance of the shorted BNC plug.

At 16 MHz and below, the MFJ-269 shows it as 0+0j. Above 16 MHz, some
reactance shows up... 2 ohms at 28 MHz, 3 ohms at 42 MHz, 4 ohms and
55 MHz, 8 ohms at 112 MHz, 10 ohms at 144 MHz, 12 ohms at 169 MHz.
The inductance of the shorted plug itself, and/or the length of the
N-to-BNC adapter on the analyzer, is having some effect. SWR remains
unreadably high: 31 up through VHF and 5 on 440. It's not the
highest-quality "short circuit" in the world, for certain, but at HF
and VHF it's close enough for our purposes here.

So... what do I conclude from this?

I conclude that at HF and VHF frequencies, if you accidentally short
your transmitter-to-antenna coaxial feedline (with a pin, nail, or a
loose strand of coax braid which "gets loose" inside a connector),
you're going to present your transmitter with an unrealistic load
(high, low, or nastily reactive) and that the power flow up the
feedline is going to stop at the short circuit.

Definatly at HF, likely at low VHF. As we get to 300mhz we don't
know but, likely not good at all.

Since no "short circuit" in the real world is going to have 0+0j
impedance, this isn't *absolutely* true, but you can probably consider
it to be *practically* true and correct at these frequencies, with
this sort of shorting.

I further conclude that with respect to the above, there's no 'magic'
about shorts which happen to occur at a quarter-wavelength distance
from the transmitter. In this situation, the transmitter will 'see'
something which behaves like a quarter-wavelength shorted stub... the
load further up the transmission line at the antenna remains
'invisible' to the transmitter.

The last two tests suggests or at lest hints something is going on and
measurable.

At high-UHF and SHF/microwave frequencies, where a direct "short" is
likely to be a significant fraction of a wavelength, then the
additional reactance of the "short" will certainly affect how the
undesired connection affects what the transmitter "sees". It may
hurt, or help (helping to "tune out" reactance from the antenna
itself), or may be completely invisible. Allison is entirely correct
on this point, under these particular conditions.

We do agree, if the conditions are unspecified are also likely
meaningless. If the conditions are specified we will likely agree.
I know it sounds picky to no end but it's about those little second
and third order things often ignored and renders answers that are
more lore than tested fact.

Allison

On Mon, 26 Dec 2005 08:04:02 -0600, "Richard Fry"
wrote:

wrote

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.
___________

Yes, and in typical configuration it is an electrical 1/4-wave coaxial
section connected in parallel with both conductors of the main transmission
line. It does not terminate the main transmission line, so this
application/example is not very relevant to the "pin through the coax"
question of the OP. And it would not result in a dead short at the carrier
frequency, no matter where it is located in the output system. Some FM
broadcast antennas also include them to supply a DC short from inner to
outer conductors of the antenna coax to provide some protection from
lightning.

A 1/4-wave shorted stub is used at frequencies as low as the MW broadcast
band. The need there is to add a deep notch at stations whose 2nd harmonic
falls in the broadcast band. This stub is used to add to the attenuation of
the already compliant 2nd harmonic level coming out of the tx, but which,
without the stub can be heard on broadcast receivers within a short distance
from the broadcast antenna site. WJR (760 kHz) is one station using this
technique.

That just strikes me as plain stoopid. At MW, such filtering would be
far better achieved by lumped elements. A quarter wave stub at such
frequencies appears impractical, unwieldy and rather expensive!
--

"What is now proved was once only imagin'd" - William Blake

wrote

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.
___________

Yes, and in typical configuration it is an electrical 1/4-wave coaxial
section connected in parallel with both conductors of the main transmission
line. It does not terminate the main transmission line, so this
application/example is not very relevant to the "pin through the coax"
question of the OP. And it would not result in a dead short at the carrier
frequency, no matter where it is located in the output system. Some FM
broadcast antennas also include them to supply a DC short from inner to
outer conductors of the antenna coax to provide some protection from
lightning.

A 1/4-wave shorted stub is used at frequencies as low as the MW broadcast
band. The need there is to add a deep notch at stations whose 2nd harmonic
falls in the broadcast band. This stub is used to add to the attenuation of
the already compliant 2nd harmonic level coming out of the tx, but which,
without the stub can be heard on broadcast receivers within a short distance
from the broadcast antenna site. WJR (760 kHz) is one station using this
technique.

RF (WJR engr in mid-1960s)

"Paul Burridge" wrote
A 1/4-wave shorted stub is used at frequencies as low as the MW broadcast
band. The need there is to add a deep notch at stations whose 2nd
harmonic
falls in the broadcast band. This stub is used to add to the attenuation
of
the already compliant 2nd harmonic level coming out of the tx, but which,
without the stub can be heard on broadcast receivers within a short
distance
from the broadcast antenna site. WJR (760 kHz) is one station using this
technique. (RF quote)

That just strikes me as plain stoopid. At MW, such filtering would be
far better achieved by lumped elements. A quarter wave stub at such
frequencies appears impractical, unwieldy and rather expensive!
____________

It also provides a low-impedance and fairly wideband path to ground for the
insulated, series-fed tower used by most broadcast stations -- which drains
off any static charges that may collect on the tower, and so reduces the
probability of lightning strikes.

Lumped elements are less effective at this.

RF

"Paul Burridge" k wrote
in message ...


That just strikes me as plain stoopid. At MW, such filtering would be
far better achieved by lumped elements. A quarter wave stub at such
frequencies appears impractical, unwieldy and rather expensive!
--


Even at 50 KW?
73, Steve, K,9.D;C'I

P.S. I suspect it is air line, no?