"Ken Bessler" wrote in message news:...

My current antenna is a modified Van Gordon "All Bander".
For reference, the stock antenna is 134' of 14g stranded
bare copper fed with 100' of 450 ohm ladder line. I cut off
half of the ladder line and replaced it with a 12 turn, 5-1/4"
rg58 coil. 20 feet of rg58 goes from the coil into the shack.
I then trimmed the antenna to 7.175 mhz.

The antenna is a flat dipole up 20'. Seems to work OK but
I've got the itch to improve it. The feed point is 12" above
the peak of my roof. I was thinking about installing a 5' tripod
and 2 ten foot masts to elevate the antenna's feed point and
convert it into an inverted V. There will be no need to add to
the feedline.


snip


Well, it's up guys! The apex is about 40' and the ends are 28'.
Comparing it to my other antennas I'm seeing about a 1-1/2 to
2 S unit improvement on 40m and more on the higher bands.

I did a SWR plot before and after changing it and here's the
sticking point: Before, I had a 2:1 range from 6.900-7.280 and
now I've got 7.000-7.300 but the minimum SWR went from
1.10:1 to 1.55:1. My Elecraft KAT2 autotuner handles that very
easily but I'm wondering what would happen if I added 12 feet
to my 50' of 450 ohm ladder line? Would the minimum SWR
get better or just move? Or both? Does it really matter? (I don't
think it matters based on what I've read here).

Ken

--
Just my 2¢... 73 es gd dx de Ken KGØWX
Grid EM17ip, Flying Pigs #-1055
Proud builder & owner of Elecraft K2 #4913

Ken Bessler wrote:

"Ken Bessler" wrote in message news:...


My current antenna is a modified Van Gordon "All Bander".
For reference, the stock antenna is 134' of 14g stranded
bare copper fed with 100' of 450 ohm ladder line. I cut off
half of the ladder line and replaced it with a 12 turn, 5-1/4"
rg58 coil. 20 feet of rg58 goes from the coil into the shack.
I then trimmed the antenna to 7.175 mhz.

The antenna is a flat dipole up 20'. Seems to work OK but
I've got the itch to improve it. The feed point is 12" above
the peak of my roof. I was thinking about installing a 5' tripod
and 2 ten foot masts to elevate the antenna's feed point and
convert it into an inverted V. There will be no need to add to
the feedline.



snip


Well, it's up guys! The apex is about 40' and the ends are 28'.
Comparing it to my other antennas I'm seeing about a 1-1/2 to
2 S unit improvement on 40m and more on the higher bands.

I did a SWR plot before and after changing it and here's the
sticking point: Before, I had a 2:1 range from 6.900-7.280 and
now I've got 7.000-7.300 but the minimum SWR went from
1.10:1 to 1.55:1. My Elecraft KAT2 autotuner handles that very
easily but I'm wondering what would happen if I added 12 feet
to my 50' of 450 ohm ladder line? Would the minimum SWR
get better or just move? Or both? Does it really matter? (I don't
think it matters based on what I've read here).

Ken

SWR has nothing to do with the length of line [assuming low loss line].
SWR is determined solely by the antenna to transmission line impedance.
[e.g. an antenna input impedance of 70 ohms and a 50 ohm cable has a SWR
of 1.4:1 ALL the time].

SWR changes as a function of antenna height above ground because of
ground effects. Don't worry about it! The ground effect has a mutual
impedance impact on the antenna. Antennas close to ground, 1/4
wavelength, tend to have lower feedpoint impedances. Antennas 1/4
wavelength high will exhibit a higher impedance that will oscillate and
converge on Ro as a function of higher height, ultimately achieving Ro
in free space.

It doesn't matter in the amateur radio world or the engineering world.
Physicists will want to understand all the variables involved in the
interest of science. That's great! But, I'm more into amateur radio and
not physics.

In article ,
Ham op wrote:
I did a SWR plot before and after changing it and here's the
sticking point: Before, I had a 2:1 range from 6.900-7.280 and
now I've got 7.000-7.300 but the minimum SWR went from
1.10:1 to 1.55:1. My Elecraft KAT2 autotuner handles that very
easily but I'm wondering what would happen if I added 12 feet
to my 50' of 450 ohm ladder line? Would the minimum SWR
get better or just move? Or both? Does it really matter? (I don't
think it matters based on what I've read here).

SWR has nothing to do with the length of line [assuming low loss line].

True in one respect, but not necessarily a complete answer in this
case (I think).

SWR is determined solely by the antenna to transmission line impedance.
[e.g. an antenna input impedance of 70 ohms and a 50 ohm cable has a SWR
of 1.4:1 ALL the time].

True, but that's not the whole story here.

If I understand the OP's setup properly, he's trimmed his antenna wire
down to be a half-wave 40-meter dipole. He's then feeding it with a
combination of ladder line, a coaxial balun, and coax to the shack.

If that's a true picture of the situation, then I would expect the SWR
at the shack end of the coax to vary quite a lot as the length of the
ladder line is changed.

Reasoning: the antenna's feedpoint impedance wouldn't be too far from
the 50-to-70 ohm range, give its resonant length and its height above
ground. This impedance will be transformed by the 450-ohm ladder
line, and the impedance at the ladder-line/coax joining point will
depend on the degree of transformation.

If the ladder line is cut to be an integral number of electrical
half-wavelengths, it would mirror the antenna feedpoint impedance to
the coax. There's be a good match, the SWR on the coax would be
somewhere between 1:1 and 1.5:1, and the rig would quite possibly be
happy without even needing its internal ATU. This is Good.

On the other hand, if the ladder line is an odd number of
quarter-wavelengths, it would transform the antenna's feedpoint
impedance up to a much higher value (in excess of 1000 ohms). This
would result in a severe mismatch at the ladder-line/coax joining
point, and a high SWR on the coax. I'd expect both high losses, and a
significant amount of RF appearing on the coax braid (the coiled-coax
choke balun wouldn't have enough choking reactance). The rig's ATU
might not be able to match this load at all. This would be Bad.

In these two extreme cases, the SWR on the ladder line would be high,
and wouldn't change appreciably as the ladder-line length was changed.
Losses in the ladder line should be minimal.

The SWR on the coax section, and the impedance presented to the rig,
would vary anywhere from "utterly benign" to "nasty and unacceptable"
depending on the length of the ladder line.

The same basic reasoning would seem to apply, I think, for any antenna
feedpoint impedance other than one which matches that of the ladder
line. Although changing the ladder-line length won't affect the SWR
on the ladder line, it can affect the SWR on the coax section
(possibly by a lot!) due to the varying transformation of the
antenna's feedpoint impedance.

This is, in a way, the converse of Cecil's "no tuner" matching
arrangment, which uses a tuned ladder-line feeder to transform a
dipole's feedpoint impedance down to 50 ohms on multiple bands by
adjusting the feeder length.

--
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!

----- Original Message -----
From: "Dave Platt"
Newsgroups: rec.radio.amateur.antenna
Sent: Tuesday, July 26, 2005 8:08 PM
Subject: Raising a 40m dipole.....It's up! Now what?

snip

If the ladder line is cut to be an integral number of electrical
half-wavelengths, it would mirror the antenna feedpoint impedance to
the coax. There's be a good match, the SWR on the coax would be
somewhere between 1:1 and 1.5:1, and the rig would quite possibly be
happy without even needing its internal ATU. This is Good.

On the other hand, if the ladder line is an odd number of
quarter-wavelengths, it would transform the antenna's feedpoint
impedance up to a much higher value (in excess of 1000 ohms). This
would result in a severe mismatch at the ladder-line/coax joining
point, and a high SWR on the coax. I'd expect both high losses, and a
significant amount of RF appearing on the coax braid (the coiled-coax
choke balun wouldn't have enough choking reactance). The rig's ATU
might not be able to match this load at all. This would be Bad.

snip

Would that explain why my tuner can only get a 2.6:1 match on 12m
with a 50 foot section of ladder line? Here's my (probably wrong) math:

((246/24.940)(0.92)5)= 45'4.5"

That's 246 / Freq x Velocity Factor x 5 (odd quarter wavelength)

On a related issue, my lowest SWR went from a broad 1.6:1 to a
narrow 1.1:1 after it stopped raining. Was that the wet asphault shingles
effecting the ladder line or just the wet ladder line? If it's the shingles,
I could elevate the line but either way, my tuner handles it. Should I care?

I've now plotted the SWR and (dry), the curve is nearly the same
as it was before I raised the antenna - it's just centered about 45 khz
higher and a bit narrower. This is with the tuner in bypass mode.

Thanks for all the help, guys!

Ken
--
Just my 2¢... 73 es gd dx de Ken KGØWX
Grid EM17ip, Flying Pigs #-1055
Proud builder & owner of Elecraft K2 #4913

Ken Bessler wrote:
((246/24.940)(0.92)5)= 45'4.5"

That's 246 / Freq x Velocity Factor x 5 (odd quarter wavelength)

Calculated values rarely ever take all the variables into account.
Virtually all calculated values have to be adjusted, i.e. cut
and tried, to achieve the desired results. What SWR do you get
when you increase the length by one foot? What SWR do you get
when you decrease the length by one foot?

On a related issue, my lowest SWR went from a broad 1.6:1 to a
narrow 1.1:1 after it stopped raining. Was that the wet asphault shingles
effecting the ladder line or just the wet ladder line? If it's the shingles,
I could elevate the line but either way, my tuner handles it. Should I care?

Do you care? :-) It is a good idea to keep ladder-line a few
inches away from anything that might adversely affect it. That
includes wet asphalt shingles.
--
73, Cecil http://www.qsl.net/w5dxp

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On Tue, 26 Jul 2005 17:01:17 -0500, "Ken Bessler"
wrote:

"Ken Bessler" wrote in message news:...

My current antenna is a modified Van Gordon "All Bander".
For reference, the stock antenna is 134' of 14g stranded
bare copper fed with 100' of 450 ohm ladder line. I cut off
half of the ladder line and replaced it with a 12 turn, 5-1/4"
rg58 coil. 20 feet of rg58 goes from the coil into the shack.
I then trimmed the antenna to 7.175 mhz.

The antenna is a flat dipole up 20'. Seems to work OK but
I've got the itch to improve it. The feed point is 12" above
the peak of my roof. I was thinking about installing a 5' tripod
and 2 ten foot masts to elevate the antenna's feed point and
convert it into an inverted V. There will be no need to add to
the feedline.


snip


Well, it's up guys! The apex is about 40' and the ends are 28'.
Comparing it to my other antennas I'm seeing about a 1-1/2 to
2 S unit improvement on 40m and more on the higher bands.

I did a SWR plot before and after changing it and here's the
sticking point: Before, I had a 2:1 range from 6.900-7.280 and
now I've got 7.000-7.300 but the minimum SWR went from
1.10:1 to 1.55:1. My Elecraft KAT2 autotuner handles that very
easily but I'm wondering what would happen if I added 12 feet
to my 50' of 450 ohm ladder line? Would the minimum SWR
get better or just move? Or both? Does it really matter? (I don't
think it matters based on what I've read here).

Ken, this is all a bit scant on detail, and can't be interpreted with
certainty, so i am probably wasting my time running the numbers.

All the following in premised on my interpretation of your rather
loose description, and is only applicable to your situation if what
you have done is *very* close to my interpretation.

I think you have said you have cut a 135' dipole down to 7Mhz, so if
that means it is a half wave long at 7MHz, the feedpoint impedance at
resonance mounted low as you have described is probably somewhere
about 60+j0 ohms.

If you transform that with 50' of ladder line (I have modelled Wireman
554), the Zin to the ladder is around 110-j310 and the ladder line has
about 0.25dB of loss. You describe a coil of RG58 that seems to be 15'
in length and a further 20' of the same. That will transform 110-j310
ohms to 5+j14 at the ATU end with about 2.5dB of loss. The RG58
operates at high VSWR (~12), and although relatively short, the loss
is IMHO unacceptable.

There will be a little tuner loss, but it will be insignificant
relative to the 2.7dB of feedline loss.

If you want to improve the configuration, the RG58 has to be a prime
target of your attention. Most of the real power you deliver to the
feedpoint will be radiated (ie conductor losses in the dipole are
relatively insignificant), the challenge is to get the power to the
feedpoint. In your case you are probably getting around 50% of the
transmitter power to the feedpoint.

The closer the dipole is to ground, the more radiation is concentrated
upwards, so if you want low angle radiation, raising the dipole height
helps.

Owen
--

On Wed, 27 Jul 2005 22:43:34 GMT, Owen wrote:

ohms to 5+j14 at the ATU end with about 2.5dB of loss. The RG58
operates at high VSWR (~12), and although relatively short, the loss
is IMHO unacceptable.

I should have added that the VSWR at the ATU end is as low as around
12, and is closer to 20 at the far end. (Knowing VSWR doesn't provide
the solution to this problem.)

Owen
--

Well, it's up guys! The apex is about 40' and the ends are 28'.
Comparing it to my other antennas I'm seeing about a 1-1/2 to
2 S unit improvement on 40m and more on the higher bands.

I did a SWR plot before and after changing it and here's the
sticking point: Before, I had a 2:1 range from 6.900-7.280 and
now I've got 7.000-7.300 but the minimum SWR went from
1.10:1 to 1.55:1. My Elecraft KAT2 autotuner handles that very
easily but I'm wondering what would happen if I added 12 feet
to my 50' of 450 ohm ladder line? Would the minimum SWR
get better or just move? Or both? Does it really matter? (I don't
think it matters based on what I've read here).

Ken

--
Just my 2¢... 73 es gd dx de Ken KGØWX
Grid EM17ip, Flying Pigs #-1055
Proud builder & owner of Elecraft K2 #4913


If it ain't broke Ben, and it's not, do not fix it!!

Butch KF5DE

"Butch Magee" wrote in message
...
Well, it's up guys! The apex is about 40' and the ends are 28'.
Comparing it to my other antennas I'm seeing about a 1-1/2 to
2 S unit improvement on 40m and more on the higher bands.

I did a SWR plot before and after changing it and here's the
sticking point: Before, I had a 2:1 range from 6.900-7.280 and
now I've got 7.000-7.300 but the minimum SWR went from
1.10:1 to 1.55:1. My Elecraft KAT2 autotuner handles that very
easily but I'm wondering what would happen if I added 12 feet
to my 50' of 450 ohm ladder line? Would the minimum SWR
get better or just move? Or both? Does it really matter? (I don't
think it matters based on what I've read here).

it doesn't matter! get on the air!

On Thu, 28 Jul 2005 23:24:52 -0000, "Dave" wrote:


"Butch Magee" wrote in message
...
Well, it's up guys! The apex is about 40' and the ends are 28'.
Comparing it to my other antennas I'm seeing about a 1-1/2 to
2 S unit improvement on 40m and more on the higher bands.

I did a SWR plot before and after changing it and here's the
sticking point: Before, I had a 2:1 range from 6.900-7.280 and
now I've got 7.000-7.300 but the minimum SWR went from
1.10:1 to 1.55:1. My Elecraft KAT2 autotuner handles that very
easily but I'm wondering what would happen if I added 12 feet
to my 50' of 450 ohm ladder line? Would the minimum SWR
get better or just move? Or both? Does it really matter? (I don't
think it matters based on what I've read here).

it doesn't matter! get on the air!


You're looking at MAYBE a three percent improvement in signal. As the
others have said, it's not broken, so why fix it?


Raymond Sirois KU2S
SysOp: The Lost Chord BBS
607-733-5745
telnet://thelostchord.dns2go.com:6023