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Old November 16th 06, 10:50 PM posted to rec.radio.amateur.antenna
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Default matching 50 ohm line to 75 ohm antenna


OK, It's a monoband LP.

I am assuming that the inter-element crossover connections are correct.

Yes, double checked the assembly instructions


The feedpoint impedance is determined primarily by the dimensions and spacing of
the boom elements. They make a two element open wire transmission line. This is
fixed by the design and is not readily tunable [adjustable]. However, secondary
tuning adjustments are possible.

Its a plug and play, no tune LPA however i feel there are some strage
circumstances with the
feed or something as it worked fine at a different location & feedline
length.

The "shorted with a bar" is a tuning stub that provides some degree of
adjustment. As stated in another post you may have a 200 ohm or possibly 300 ohm
feedpoint impedance [VSWR 4:1]. Please tell us what VSWR value you are seeing.
A VSWR of 4:1 indicates a 200 ohms design; 6:1 indicates a 300 ohm design, etc.

No its a fixed bar and direct 50 connection.

Check the dimensions of the tuning stub for length as described by the
manufacturer.

No tunning

A LP is a balanced driven array. It should NOT be fed directly with coax [as you
describe]. Direct feed causes a current imbalance and currents on the outside
braid of the coax. You need either a choke balun or a ferrite based 6 meter balun.

No balun is needed however there maybe something strange going on so I
will try a coax
balun and try a fiberglass mounting pole, bottom line is i moved this
LPA about 30 foot, changed the polarity from vert to hor, same type of
feedline was used but instead of 70' its 35'long otherwize everythiong
stayed the same.

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Old November 16th 06, 11:58 PM posted to rec.radio.amateur.antenna
Wes Wes is offline
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Default matching 50 ohm line to 75 ohm antenna


Dave wrote:
[snip]

A LP is a balanced driven array. It should NOT be fed directly with coax [as you
describe]. Direct feed causes a current imbalance and currents on the outside
braid of the coax. You need either a choke balun or a ferrite based 6 meter balun.


Actually if the coax is fed through one half of the boom with the outer
conductor attached to that side and the center conductor connected to
the other half of the boom you don't need a balun.

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Old November 18th 06, 02:31 PM posted to rec.radio.amateur.antenna
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Default matching 50 ohm line to 75 ohm antenna


Tam/WB2TT wrote:
"N4aeq" wrote in message
oups.com...

Dave wrote:
N4aeq wrote:

Im trying to match 50 ohm coax (lmr400) with a log peroidic which
should have a 75 ohm feedpoint
the SWR is flat across the 6 m band but high. Im thinking i need a
better match but not sure
where to start.


You have not stated which LP you are using. So, my comments are general
in nature.

The feedpoint impedance of a LP does vary as a function of frequency and
the log
characteristics of the elements, their spacing, and the boom geometry.
The
feedpoint impedance will cycle between a low to high value and generally
remain
below 1.7:1.

Example: my HF LP [Tennadyne T8] can have any feedpoint impedance between
29.4
and 85 ohms as I tune from 13.5 MHz to 30 MHz and it will have numerous
excursions within the frequency range. That is the Physics of LP design.

Since the feedpoint impedance is a variable it is not easily matched to a
standard coax line.

Now, a VSWR of 1.7:1 relates to approximately 93% of the rigs power being
coupled to the antenna.

To your comment. What is the 6 meter VSWR?

If it is less than 2:1 [90% coupling to the antenna] then forget it!


Im using a double boom LPA, 50 coax connected to the front with shield
to one boom and
cc to the other. The back of the boom is shorted with a bar, it is a
monoband 50-54mhz
5 element. Swr nowhere near 2:1 (i wish) and i have checked the line
so its ok so maybe
i should have ask how do you adjust the impeadance on a double boom lpa?

I still think it is a 200 Ohm antenna.

Tam


On a double boom LPA ithe nominal value can be around 50 ohms, It
depends on the spacing of the booms. Bill Orr has a design for 6 meters
that used 3 or 4 elements in an LPA and then added a few parasitic
elements. I tried this antenna and mine had no noticeable improvement
over my 4 element Yagi

Jimmie D

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Old November 18th 06, 02:31 PM posted to rec.radio.amateur.antenna
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Default matching 50 ohm line to 75 ohm antenna


Tam/WB2TT wrote:
"N4aeq" wrote in message
oups.com...

Dave wrote:
N4aeq wrote:

Im trying to match 50 ohm coax (lmr400) with a log peroidic which
should have a 75 ohm feedpoint
the SWR is flat across the 6 m band but high. Im thinking i need a
better match but not sure
where to start.


You have not stated which LP you are using. So, my comments are general
in nature.

The feedpoint impedance of a LP does vary as a function of frequency and
the log
characteristics of the elements, their spacing, and the boom geometry.
The
feedpoint impedance will cycle between a low to high value and generally
remain
below 1.7:1.

Example: my HF LP [Tennadyne T8] can have any feedpoint impedance between
29.4
and 85 ohms as I tune from 13.5 MHz to 30 MHz and it will have numerous
excursions within the frequency range. That is the Physics of LP design.

Since the feedpoint impedance is a variable it is not easily matched to a
standard coax line.

Now, a VSWR of 1.7:1 relates to approximately 93% of the rigs power being
coupled to the antenna.

To your comment. What is the 6 meter VSWR?

If it is less than 2:1 [90% coupling to the antenna] then forget it!


Im using a double boom LPA, 50 coax connected to the front with shield
to one boom and
cc to the other. The back of the boom is shorted with a bar, it is a
monoband 50-54mhz
5 element. Swr nowhere near 2:1 (i wish) and i have checked the line
so its ok so maybe
i should have ask how do you adjust the impeadance on a double boom lpa?

I still think it is a 200 Ohm antenna.

Tam


On a double boom LPA ithe nominal value can be around 50 ohms, It
depends on the spacing of the booms. Bill Orr has a design for 6 meters
that used 3 or 4 elements in an LPA and then added a few parasitic
elements. I tried this antenna and mine had no noticeable improvement
over my 4 element Yagi

Jimmie D

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Old November 18th 06, 02:39 PM posted to rec.radio.amateur.antenna
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Posts: 444
Default matching 50 ohm line to 75 ohm antenna

JIMMIE wrote:

SNIPPED

On a double boom LPA ithe nominal value can be around 50 ohms, It
depends on the spacing of the booms. Bill Orr has a design for 6 meters
that used 3 or 4 elements in an LPA and then added a few parasitic
elements. I tried this antenna and mine had no noticeable improvement
over my 4 element Yagi

Jimmie D


Jimmie, I would expect no improvement in GAIN over a four element Yagi.

But how does the VSWR compare between the two antennas when measured
continuously from 50.0 to 54.0 MHz? I expect the LP will have a more uniform
VSWR. That is why people choose LPs. Namely to get a 'flatter' VSWR response.

/s/ DD



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Old November 21st 06, 02:39 AM posted to rec.radio.amateur.antenna
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Posts: 296
Default matching 50 ohm line to 75 ohm antenna


"Dave" wrote in message
. ..
JIMMIE wrote:

SNIPPED

On a double boom LPA ithe nominal value can be around 50 ohms, It
depends on the spacing of the booms. Bill Orr has a design for 6 meters
that used 3 or 4 elements in an LPA and then added a few parasitic
elements. I tried this antenna and mine had no noticeable improvement
over my 4 element Yagi Jimmie D


Jimmie, I would expect no improvement in GAIN over a four element Yagi.

But how does the VSWR compare between the two antennas when measured
continuously from 50.0 to 54.0 MHz? I expect the LP will have a more
uniform VSWR. That is why people choose LPs. Namely to get a 'flatter'
VSWR response.

/s/ DD

I was never really interested in covering the whole band , just the first
1.5 Mhz or so. The SWR was comparable in this area on both antennas. The
Yagi was easier and cheaper to build.


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