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1.5 to 1 matching section......




 
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RolandSWL
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« on: May 06, 2023, 10:43:38 AM »

Hi All,

I need to match a 10 meter dipole to 50 ohm feed line. I don't want to spend $50+ on a balun. What is the procedure for accomplishing this using sections of 75 an 50 ohm line?
Thanks,Roland
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w7fox
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« Reply #1 on: May 06, 2023, 12:12:45 PM »

You can consult your antenna books for matching with sections of transmission lines, but bear in mind the resistance of a dipole depends on its height above ground and any nearby objects.  Matching with coax sections is not a balanced feed, if that is important to you.  Back in the day, we just hooked coax to the dipole and adjusted the Pi network.
I once had a 15 meter dipole strung around my dorm room ceiling using a gamma match.  No balun necessary and very adjustable.  Experimenting with antennas is great fun, enjoy.
Best regards,
Fox
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ka1bwo
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« Reply #2 on: May 07, 2023, 02:03:07 AM »

Hi All,

I need to match a 10 meter dipole to 50 ohm feed line. I don't want to spend $50+ on a balun. What is the procedure for accomplishing this using sections of 75 an 50 ohm line?
Thanks,Roland
Hi Roland,
You will be able to use a dipole for 10 meters for a 50 ohm system without a matching network. The antenna length L= 468 / f (MHz) and height slightly less than .2 wavelength. Maybe you can't meet this criterion and have restrictions?
Joe


* dipole-impedance vs height.jpg (15.89 KB, 300x285 - viewed 94 times.)
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WBear2GCR
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« Reply #3 on: May 07, 2023, 10:09:05 AM »

no balun required.

I've never ever used one.

Just trim the ant once it is hung, you can let the ends hang down, a foot or so... then just
loop them back up on itself to tune.

10m is very wide, so you can't precisely match the entire band anyhow.
Not a real problem to use a "tuner" to improve how a solid state rig without an internal
tuner "sees" the SWR. For a receiver, non-issue.
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W1ITT
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« Reply #4 on: May 07, 2023, 10:38:58 AM »

To more directly answer Roland's question, there is indeed a method to use intervening line sections of another impedance to match a load.  But first, one needs to plot the load on a Smith chart so you know what your initial R+jX is.  Knowing the K-factor (impedance of the intervening section) you can then rotate the load around to determine the proper length and position of the matching section  I used to do this all day long matching FM  and TV antennas.  We used "slugs", actually something like a shaft collar clamped onto the inner conductor of rigid coaxial line.  Obviously, it was a lower impedance slug, but it's all just arithmetic.  I computed the slugs graphically using a Smith Chart.  (Once I phoned the company to order more Smith charts and talked for a bit with Philip Smith, a great engineer and a fine gentleman,)  Now I am sure there is software that will do the heavy lifting for you.   ARRRL had a Smith chart calculating program for a while that might suffice. It was manual and a bit clunky. It might have been called Microsmith or something similar, if memory serves.
But, for ham radio, 1.5 to one isn't so bad.  I confess to being an SWR junkie, spending too much time to make it unnecessarily good.
73 de Norm W1ITT
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K1JJ
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« Reply #5 on: May 07, 2023, 10:53:11 AM »

Hi Roland,

Ten meters is an easy band to make a coaxial balun for.  Just wind up 3-4 turns of the feedline coax at the dipole feedpoint so that none of the turns overlap.  In addition, if you have any RF ferrite bead cores hanging around, slip them onto the coax at the feedpoint.  Depending on the cores, only a few are needed for a high impedance on 10M.

This should eliminate any feedline radiation, assumimg you had any in the first place.

And for the more advanced, looking for a perfect match, you really want to know the true impedance at the dipole feedpoint. (resonance)  Pick your favorite spot on the band.  (29.00 MHz?)  The most accurate way of tuning the antenna is using a true 1/2 wavelength multiple of coax feedline (using 0.66 velocity factor)   OR  temporarily put the impedance meter at the dipole center without a feedline for measurments.  

492/29 MHz * .66 =  11.1'  or 22.4'  or  34.4' coax feedline length, etc. Trim the dipole ends for the best match.  
Both methods will give a TRUE impedance reading.  But when using a random length feedline, the reading at the transmitter will be inaccurate (not showing the dipole feedpoint) unless when the feedline impedance is the same as the load. (50 + j0 ohms).

IE, it can be tricky unless the feedline is a true 1/2 wave multiple, or there is no feedline at all, IE, measured at the dipole center input.

And yes, as Norm said above,  intervening line sections of another impedance to match a load works FB too.  A section of 75 ohm coax at or near the dipole feedpoint of a certain length can usually produce a desired 1:1 match for 50 ohm coax.

Some of this is more important for Yagis and stacked arrays and not as critcal for simple dipoles. But if we can make the feedline radiation and match near-perfect on a dipole, why not? A dB here, a dB there - they all add up.

T  
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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wa2tak
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« Reply #6 on: May 07, 2023, 10:55:42 AM »

Why not simply resonate it?
And look at the SWR 3 to 3 bandwidth.
If necessary use an antenna tuner.
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RolandSWL
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« Reply #7 on: May 07, 2023, 06:21:18 PM »

Thank you kindly for your replies. I think I found the answer here: http://amfone.net/Amforum/index.php?topic=19648.0;wap2
You are all the best.
Roland...........
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K1JJ
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« Reply #8 on: May 07, 2023, 08:33:53 PM »

Thank you kindly for your replies. I think I found the answer here: http://amfone.net/Amforum/index.php?topic=19648.0;wap2
You are all the best.
Roland...........

FB, Roland!

I forgot that I wrote about the 1/12 wave technique years ago....

The pictoral diagram of the technique has a broken link, so here is a description for the archives future use:

When using a 50 ohm antenna matched to a 75 ohm feedline, 1.5 to 1 step down:    50 ohm antenna >  1/12 wave 75 ohm coax > 1/12 wave 50 ohm coax > 75 ohm feedline to TX

When using a 75 ohm antenna matched to a 50 ohm feedline,  1 to 1.5 step up:   75 ohm antenna >  1/12 wave 50 ohm coax > 1/12 wave 75 ohm coax > 50 ohm feedline to TX

Example on 10M for 1/12 wavelength RG-213 coax section:  984 / 29 MHz  * 0.0815 * 0 .66 vel fact = 1.825' = 21.9"

I've used this technique on many antennas in the past. The good thing is that even on 75M, the sections are very short. (only 13.9')    You can even wind the matching coax section(s) onto an ABS 4" diameter tube to double as the balun/RF choke at the feedpoint.  A dipole never had it so good.  Works like a charm. EMP will not hurt it like ferrite. Let us know how it works out!

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
W1ITT
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« Reply #9 on: May 08, 2023, 08:11:35 AM »

The 1/12 wave section method is a good expedient matching system but if there is much reactance at the feedpoint of the antenna, the numbers will be a bit off.  And any  simple matching device with frequency dependent elements will tend to reduce SWR bandwidth, which may be important on a wide allocation such as 10 meters. 
Someone already posted the chart of resistive component versus height for a half wave dipole.  Another trick is to set up the dipole as an inverted V which can push things pretty close to 50 ohms.  Lotsa ways to skin that cat.
73 de Norm W1ITT
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WD8BIL
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« Reply #10 on: May 08, 2023, 08:31:22 AM »

A 1.5 SWR isn't much to worry about anyways. You'll see no difference on recieve and the reflected Tx power is only 4%.

That said, the easiest way is a length of 75 Ohm, typically 0.66 VF, 5' 8".
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RolandSWL
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« Reply #11 on: May 08, 2023, 10:02:58 AM »

Does it make any difference that it's a vertical dipole?
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WD8BIL
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« Reply #12 on: May 08, 2023, 12:48:31 PM »

"Does it make any difference that it's a vertical dipole?"

Nope! In fact, that's the same thing I'm doing with my 6 meter repeater vertical dipole. Works great.

To figure the quarter wave section, Roland, take  246/freq (I used 28.5MHz for yours) X coax velocity factor. So 246/28.5 X 0.66 = 5.696ft.

Of course if you use a coax with a different velocity factor you'll need to use that in the equation instead of the 0.66
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KD6VXI
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« Reply #13 on: May 09, 2023, 06:08:33 AM »

Vertical dipole is most easily made with a ferrite choke and a piece of coax.

Use a choke at a quarter wave back from the end of the coax.  The coax shield acts as half the dipole.

Basically, a Shakespeare 11 meter 'Big Stick'.

Of course, if you don't have a ferrite ring, this isn't plumbers delight.  However, it gets rid of the need to run the coax at a right angle from the antenna.

--Shane
WP2ASS / ex KD6VXI
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RolandSWL
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« Reply #14 on: May 09, 2023, 01:23:04 PM »

All good information. Time to experiment.
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