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matching 50 ohm line to 75 ohm antenna



 
 
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  #1  
Old November 15th 06, 04:44 PM posted to rec.radio.amateur.antenna
n4aeq
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Posts: 14
Default matching 50 ohm line to 75 ohm antenna

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.

  #2  
Old November 15th 06, 05:36 PM posted to rec.radio.amateur.antenna
Tam/WB2TT
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Posts: 125
Default matching 50 ohm line to 75 ohm antenna


"N4aeq" wrote in message
ps.com...
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.

I can't remember the numbers, but there is a procedure in either the ARRL
Handbook, or antenna book for doing this. Basically, you have a short piece
of 50 Ohm coax, a short piece of 75 Ohm coax, and then an arbitrary length
of 50 Ohm transmission line.

If the antenna is 75 Ohms, your SWR on the 50 Ohm line should be no more
than 1.5:1. Are you sure you don't need a 4:1 balun? If your SWR is around 3
or 4, I would try the balun; easy neough to make out of a 1/2 wave piece of
coax.

Tam/WB2TT


  #3  
Old November 15th 06, 06:06 PM posted to rec.radio.amateur.antenna
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Posts: 18
Default matching 50 ohm line to 75 ohm antenna

Tam/WB2TT wrote:

I can't remember the numbers, but there is a procedure in either the ARRL
Handbook, or antenna book for doing this. Basically, you have a short piece
of 50 Ohm coax, a short piece of 75 Ohm coax, and then an arbitrary length
of 50 Ohm transmission line.


You are thinking of a nonsynchronous balun described he

http://www.k1ttt.net/technote/matching.html#k1xx

HOWEVER, this only works for one band (i.e. a monobander...not a
log-periodic covering multiple bands). If the antenna is 75 ohms, why
not simply use 75 ohm coax to the shack and then a tuner to convert to
50 ohms? Seems much simpler and you can normally get free 75 ohm
hardline for the asking at your local cable company (i.e. reel ends).

73, Bill W4ZV

  #4  
Old November 15th 06, 07:08 PM posted to rec.radio.amateur.antenna
Dave
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Posts: 444
Default matching 50 ohm line to 75 ohm antenna

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!



  #5  
Old November 15th 06, 07:49 PM posted to rec.radio.amateur.antenna
Cecil Moore
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Posts: 1,614
Default matching 50 ohm line to 75 ohm antenna

Tam/WB2TT wrote:
I can't remember the numbers, but there is a procedure in either the ARRL
Handbook, or antenna book for doing this. Basically, you have a short piece
of 50 Ohm coax, a short piece of 75 Ohm coax, and then an arbitrary length
of 50 Ohm transmission line.


1/4WL of RG62 followed by 1/4WL of RG59 will give a good
match from 75 ohms to 50 ohms.
--
73, Cecil http://www.w5dxp.com
  #6  
Old November 15th 06, 08:03 PM posted to rec.radio.amateur.antenna
n4aeq
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Posts: 14
Default matching 50 ohm line to 75 ohm antenna


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?

  #7  
Old November 15th 06, 10:00 PM posted to rec.radio.amateur.antenna
Tam/WB2TT
external usenet poster
 
Posts: 125
Default matching 50 ohm line to 75 ohm antenna


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

I can't remember the numbers, but there is a procedure in either the ARRL
Handbook, or antenna book for doing this. Basically, you have a short
piece
of 50 Ohm coax, a short piece of 75 Ohm coax, and then an arbitrary
length
of 50 Ohm transmission line.


You are thinking of a nonsynchronous balun described he

http://www.k1ttt.net/technote/matching.html#k1xx

HOWEVER, this only works for one band (i.e. a monobander...not a
log-periodic covering multiple bands). If the antenna is 75 ohms, why
not simply use 75 ohm coax to the shack and then a tuner to convert to
50 ohms? Seems much simpler and you can normally get free 75 ohm
hardline for the asking at your local cable company (i.e. reel ends).

73, Bill W4ZV

He was talking about 6 meters.

Tam


  #8  
Old November 15th 06, 10:03 PM posted to rec.radio.amateur.antenna
Richard Clark
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Posts: 2,951
Default matching 50 ohm line to 75 ohm antenna

On 15 Nov 2006 12:03:12 -0800, "N4aeq" wrote:

You have not stated which LP you are using.


Hi OM,

You never really answered this for Dave.

Im using a double boom LPA,


50 coax connected to the front with shield
to one boom and cc to the other.


Do you have any feedline decoupling/choking?

The back of the boom is shorted with a bar,


That should be a hairpin matching element.

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?


The point of a Log Periodic is you are not expected to tune it. The
tune is built into the design as long as it is up high enough (and
probably even if it isn't).

If you SWR is so wildly off, you have a wild problem (feedline is
undecoupled is my guess).

73's
Richard Clark, KB7QHC
  #9  
Old November 15th 06, 10:05 PM posted to rec.radio.amateur.antenna
Tam/WB2TT
external usenet poster
 
Posts: 125
Default matching 50 ohm line to 75 ohm antenna


"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


  #10  
Old November 15th 06, 11:26 PM posted to rec.radio.amateur.antenna
Dave
external usenet poster
 
Posts: 444
Default matching 50 ohm line to 75 ohm antenna

N4aeq wrote:


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?


OK, It's a monoband LP.

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

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.

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.

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

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.


 




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