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70 ohm dipole to 50 ohm feed line question



 
 
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  #1  
Old October 2nd 03, 06:22 PM
Tom Sedlack
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Posts: n/a
Default 70 ohm dipole to 50 ohm feed line question

I asked this before and was told that for relatively short runs (under 50
ft), using 50 ohm coax to feed dipoles is fine.

I keep coming back to this in my head because I model a dipole and I get a
VSWR of 1.5 using 50 ohm feed and I get 1:1 if I model with 75 ohm feed.

My plan for my little antenna farm is to have a run of 50 ohm coax to a
remote antenna switch. Create a few dipoles and feed them with 75 ohm coax.
I saw an equation someplace that, I believe, gave the length of 75 ohm feed
required to transform the impedance to 50 ohms at a given frequency. I would
cut the cable to this length then connect to the 50 ohm antenna switch.

246 x Velocity Factor
L = -----------------------------
Freq


Does all this make sense? Am I going overboard (a curse of trying to be a
perfectionist)? I don't want to use a tuner because; 1 - I don't own one
and 2 - they waste power (I want to try QRPish operation).

Also, if the above is correct, would I still need to put a current choke
(coil of coax) or would the "fact" that the feed and dipole are matched,
there wouldn't be RF on the jacket?

My head is starting to hurt........


Tom



  #2  
Old October 2nd 03, 06:58 PM
Cecil Moore
Guest
 
Posts: n/a
Default

Tom Sedlack wrote:
I don't want to use a tuner because; 1 - I don't own one
and 2 - they waste power (I want to try QRPish operation).


Check out my webpage for a method of avoiding a conventional
antenna tuner while using low-loss ladder-line.
--
73, Cecil, http://www.qsl.net/w5dxp

  #3  
Old October 2nd 03, 08:47 PM
KA9CAR
Guest
 
Posts: n/a
Default

My Quad is a 5 band. It ranges from 112 ohms down to 75 ohms on various
bands.

There is 50 ohm cable from my shack to the remote switch (a set of relays).

There are "series section transformers" That is lengths of line that are
not either 50 ohm, or the impeadence of the antenna, which convert the
impeadence of the antenna to something near 50 ohms. I think this is what
you are trying to do.

You need a piece of coax that is the "geometric mean" of the two
impeadences, and 1/4 wave long at the operating frequency.
OR
You need a common value of coax, such as 63 ohm (a standard older computer
cable, used for matching some UHF antennas) Then use the appropriate math
(beyond me) or modeling program to tell you what length to make the line.

Check out matchsec.exe
at
http://www.btinternet.com/~g4fgq.regp/

KA9CAR


"Cecil Moore" wrote in message
...
Tom Sedlack wrote:
I don't want to use a tuner because; 1 - I don't own one
and 2 - they waste power (I want to try QRPish operation).


Check out my webpage for a method of avoiding a conventional
antenna tuner while using low-loss ladder-line.
--
73, Cecil, http://www.qsl.net/w5dxp



  #4  
Old October 3rd 03, 12:26 AM
Tarmo Tammaru
Guest
 
Posts: n/a
Default

Tom,

There is a way of doing it, using two matching sections. It is described in
either the ARRL Handbook, or Antenna Manual. It takes a length of 50 Ohm
coax (NOT 1/4 wave) + a length of 75 Ohm coax (NOT 1/4 wave) to match to 50
Ohms. If I run across it, I will post it.

Before you do all that, put up the antenna and feed it with 50 Ohm coax.
Measure the SWR. Often the dipole impedance is closer to 50 than 75 Ohms,
and it is not worth doing anything about it, especially an inverted V.
Certainly, don't do anything if your radio has a built in antenna tuner.

Tam/WB2TT
"Tom Sedlack" wrote in message
...
I asked this before and was told that for relatively short runs (under 50
ft), using 50 ohm coax to feed dipoles is fine.

I keep coming back to this in my head because I model a dipole and I get a
VSWR of 1.5 using 50 ohm feed and I get 1:1 if I model with 75 ohm feed.

My plan for my little antenna farm is to have a run of 50 ohm coax to a
remote antenna switch. Create a few dipoles and feed them with 75 ohm

coax.
I saw an equation someplace that, I believe, gave the length of 75 ohm

feed
required to transform the impedance to 50 ohms at a given frequency. I

would
cut the cable to this length then connect to the 50 ohm antenna switch.

246 x Velocity Factor
L = -----------------------------
Freq


Does all this make sense? Am I going overboard (a curse of trying to be a
perfectionist)? I don't want to use a tuner because; 1 - I don't own one
and 2 - they waste power (I want to try QRPish operation).

Also, if the above is correct, would I still need to put a current choke
(coil of coax) or would the "fact" that the feed and dipole are matched,
there wouldn't be RF on the jacket?

My head is starting to hurt........


Tom





  #5  
Old October 3rd 03, 01:59 PM
Tom Sedlack
Guest
 
Posts: n/a
Default

Thanks to all who have responded!

I have ordered the Antenna Manual and will take a peak "to see what they
know" ;-)

As a vast majority of dipoles have been successfully fed with 50 ohm cable,
I'm sure I'll do the same thing. I was looking at this more from the
theoretical side in that it just seems to make sense to try and change the
impedance. All I read states that the dipole will have a characteristic
impedance of 70 ohms, the coax will have a characteristic impedance of 50
ohms. The best possible VSWR is 1.5:1 which equates to about 14 dB return
loss (this is the unit of measure I am most familiar with). 14 dB is good,
but in my line of work, I usually see better then 25 dB (VSWR 1.12:1).
It's probably overkill on my part but the experimenter in me may give this a
whirl.

Tom



"Tarmo Tammaru" wrote in message
...
Tom,

There is a way of doing it, using two matching sections. It is described

in
either the ARRL Handbook, or Antenna Manual. It takes a length of 50 Ohm
coax (NOT 1/4 wave) + a length of 75 Ohm coax (NOT 1/4 wave) to match to

50
Ohms. If I run across it, I will post it.

Before you do all that, put up the antenna and feed it with 50 Ohm coax.
Measure the SWR. Often the dipole impedance is closer to 50 than 75 Ohms,
and it is not worth doing anything about it, especially an inverted V.
Certainly, don't do anything if your radio has a built in antenna tuner.

Tam/WB2TT
"Tom Sedlack" wrote in message
...
I asked this before and was told that for relatively short runs (under

50
ft), using 50 ohm coax to feed dipoles is fine.

I keep coming back to this in my head because I model a dipole and I get

a
VSWR of 1.5 using 50 ohm feed and I get 1:1 if I model with 75 ohm feed.

My plan for my little antenna farm is to have a run of 50 ohm coax to a
remote antenna switch. Create a few dipoles and feed them with 75 ohm

coax.
I saw an equation someplace that, I believe, gave the length of 75 ohm

feed
required to transform the impedance to 50 ohms at a given frequency. I

would
cut the cable to this length then connect to the 50 ohm antenna switch.

246 x Velocity Factor
L = -----------------------------
Freq


Does all this make sense? Am I going overboard (a curse of trying to be

a
perfectionist)? I don't want to use a tuner because; 1 - I don't own

one
and 2 - they waste power (I want to try QRPish operation).

Also, if the above is correct, would I still need to put a current choke
(coil of coax) or would the "fact" that the feed and dipole are matched,
there wouldn't be RF on the jacket?

My head is starting to hurt........


Tom




  #6  
Old October 4th 03, 01:41 AM
Viðarr
Guest
 
Posts: n/a
Default

Measure the SWR. Often the dipole impedance is closer to 50 than 75 Ohms,
and it is not worth doing anything about it, especially an inverted V.


If I remember correctly, an article on Inverted Vs a long time ago in _QST_
reported that an inverted V with its element at 45° from the horizontal (or
from the vertical!) had a 50-Ohm impedance, and a horizontal and vertical
radiation pattern kind of midway between a vertical and a dipole. I fed my
40m inverted V with 45° element angles with 50-Ohm coax. It had a
design-frequency SWR of ~1.1. SWR went up to ~1.5 at the CW high band edge.

NØKF


"Tarmo Tammaru" wrote in message
...
Tom,

There is a way of doing it, using two matching sections. It is described

in
either the ARRL Handbook, or Antenna Manual. It takes a length of 50 Ohm
coax (NOT 1/4 wave) + a length of 75 Ohm coax (NOT 1/4 wave) to match to

50
Ohms. If I run across it, I will post it.

Before you do all that, put up the antenna and feed it with 50 Ohm coax.
Measure the SWR. Often the dipole impedance is closer to 50 than 75 Ohms,
and it is not worth doing anything about it, especially an inverted V.
Certainly, don't do anything if your radio has a built in antenna tuner.

Tam/WB2TT
"Tom Sedlack" wrote in message
...
I asked this before and was told that for relatively short runs (under

50
ft), using 50 ohm coax to feed dipoles is fine.

I keep coming back to this in my head because I model a dipole and I get

a
VSWR of 1.5 using 50 ohm feed and I get 1:1 if I model with 75 ohm feed.

My plan for my little antenna farm is to have a run of 50 ohm coax to a
remote antenna switch. Create a few dipoles and feed them with 75 ohm

coax.
I saw an equation someplace that, I believe, gave the length of 75 ohm

feed
required to transform the impedance to 50 ohms at a given frequency. I

would
cut the cable to this length then connect to the 50 ohm antenna switch.

246 x Velocity Factor
L = -----------------------------
Freq


Does all this make sense? Am I going overboard (a curse of trying to be

a
perfectionist)? I don't want to use a tuner because; 1 - I don't own

one
and 2 - they waste power (I want to try QRPish operation).

Also, if the above is correct, would I still need to put a current choke
(coil of coax) or would the "fact" that the feed and dipole are matched,
there wouldn't be RF on the jacket?

My head is starting to hurt........


Tom






  #7  
Old October 4th 03, 01:44 AM
Tarmo Tammaru
Guest
 
Posts: n/a
Default

Tom,

I found the procedure in the Antenna Manual for what you want to do. In the
19 th edition, it is listed in the index under "Series Section
Transformers", and is on page 26-5. Lot of terms in the formula, but nothing
fancier than an arctangent.

For a 70 Ohm load, you calculate the SWR for a 50 Ohm system as 1.4. The
MINIMUM impedance for the second coax is
50*SQRT(1.4)=59.16. So, 75 Ohm coax will work. The formulas will give you
the lengths for L1 and L2, where L1 is 50 Ohm and L2 is 75 Ohm coax.

Have fun when you get the book!

If you can't wait for the book, here are the formulas

n=Z1/Z0, where Z1 would be 75.

r=RL/Z0

x=XL/Z0

num= (r-1)^2 +x^2

den=r(n-(1/n))^2 -(r-1)^2 -x^2

B=SQRT(num/den)
L2=ARCTAN(B)

A={B(n-(r/n))+x}/{r + xnB -1}
L1=ARCTAN(A)

I don't guarantee error free typing.

Tam/WB2TT


  #8  
Old October 4th 03, 02:23 AM
Tarmo Tammaru
Guest
 
Posts: n/a
Default

The length will be in electrical degrees, where 360 deg = 1 wavelength.


  #9  
Old October 5th 03, 12:08 PM
Roy Lewallen
Guest
 
Posts: n/a
Default

The first thing you should consider is why it's important to you to
achieve such a good impedance match. I'm all in favor of stimulating
intellectual exercises, but that's all it is. It won't help your signal
by any measurable amount.

And no, a dipole doesn't have a feedpoint impedance of 70 ohms. In free
space, if it's very, very thin, resonant, and perfectly straight, it
will have an impedance close to that. But you can't construct one in
free space. Over ground, expect anything from about 50 to 100 ohms
depending on height and diameter. And that's only at the single
frequency at which it's resonant. QSY a few kHz, and there goes all your
work down the drain. But have fun. It'll be educational. And if you're
like so many others, you'll be *positive* that all the work made your
signal get out better. Not because of low SWR though, but because of a
phenomenon well known to medicine -- the placebo effect.

Roy Lewallen, W7EL

Tom Sedlack wrote:
Thanks to all who have responded!

I have ordered the Antenna Manual and will take a peak "to see what they
know" ;-)

As a vast majority of dipoles have been successfully fed with 50 ohm cable,
I'm sure I'll do the same thing. I was looking at this more from the
theoretical side in that it just seems to make sense to try and change the
impedance. All I read states that the dipole will have a characteristic
impedance of 70 ohms, the coax will have a characteristic impedance of 50
ohms. The best possible VSWR is 1.5:1 which equates to about 14 dB return
loss (this is the unit of measure I am most familiar with). 14 dB is good,
but in my line of work, I usually see better then 25 dB (VSWR 1.12:1).
It's probably overkill on my part but the experimenter in me may give this a
whirl.

Tom



"Tarmo Tammaru" wrote in message
...

Tom,

There is a way of doing it, using two matching sections. It is described


in

either the ARRL Handbook, or Antenna Manual. It takes a length of 50 Ohm
coax (NOT 1/4 wave) + a length of 75 Ohm coax (NOT 1/4 wave) to match to


50

Ohms. If I run across it, I will post it.

Before you do all that, put up the antenna and feed it with 50 Ohm coax.
Measure the SWR. Often the dipole impedance is closer to 50 than 75 Ohms,
and it is not worth doing anything about it, especially an inverted V.
Certainly, don't do anything if your radio has a built in antenna tuner.

Tam/WB2TT
"Tom Sedlack" wrote in message
...

I asked this before and was told that for relatively short runs (under


50

ft), using 50 ohm coax to feed dipoles is fine.

I keep coming back to this in my head because I model a dipole and I get


a

VSWR of 1.5 using 50 ohm feed and I get 1:1 if I model with 75 ohm feed.

My plan for my little antenna farm is to have a run of 50 ohm coax to a
remote antenna switch. Create a few dipoles and feed them with 75 ohm


coax.

I saw an equation someplace that, I believe, gave the length of 75 ohm


feed

required to transform the impedance to 50 ohms at a given frequency. I


would

cut the cable to this length then connect to the 50 ohm antenna switch.

246 x Velocity Factor
L = -----------------------------
Freq


Does all this make sense? Am I going overboard (a curse of trying to be


a

perfectionist)? I don't want to use a tuner because; 1 - I don't own


one

and 2 - they waste power (I want to try QRPish operation).

Also, if the above is correct, would I still need to put a current choke
(coil of coax) or would the "fact" that the feed and dipole are matched,
there wouldn't be RF on the jacket?

My head is starting to hurt........


Tom






  #10  
Old October 5th 03, 01:12 PM
Ralph Mowery
Guest
 
Posts: n/a
Default


And no, a dipole doesn't have a feedpoint impedance of 70 ohms. In free
space, if it's very, very thin, resonant, and perfectly straight, it
will have an impedance close to that. But you can't construct one in
free space. Over ground, expect anything from about 50 to 100 ohms
depending on height and diameter. And that's only at the single
frequency at which it's resonant. QSY a few kHz, and there goes all your


YOu are correct on the impedance of a real dipole. My 80 meter dipole is up
about 30 feet on each end but sags to about 20 feet in the middle. It is
almost 1:1 at the frequency I cut it for . That makes it closer to 50 ohms
than it is to 70 ohms.



 




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