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Author Topic: 6 meter dipole feed line....  (Read 1733 times)
K7MDO
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Posts: 325




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« on: September 25, 2008, 01:36:34 PM »

OK, here we go again... I recently put up a 6 meter 1/2 lambda dipole which is simply fed in the center with 50 ohm coax.... 18 feet long.... I hooked it to the proper antenna input on the IC-756... with internal antenna tuner... switched to 51 mHz and guess what... no match... nowehere in the band can I get the tuner to find a match with that dipole.  I receive OK on it as I found a local repeater coming in well.

Are there any thoughts out there that I may be violating the feed line length with the coax and that that may make the simple dipole too high of an SWR to make a match?

73, K7MDO
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N1OU
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« Reply #1 on: September 25, 2008, 02:26:58 PM »

A six meter dipole is going to wind up in the area of nine feet long.  468/Freq in Mhz.  I didn't run your excess length through EXNEC but I'd guess that's the problem, not the feedline length.
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WB2WIK
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« Reply #2 on: September 25, 2008, 02:30:37 PM »

A six meter dipole for 51 MHz should be 9'2" long.

18' is a full wavelength, that'll never match anything.

WB2WIK/6
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M0JHA
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« Reply #3 on: September 25, 2008, 02:37:46 PM »

Hello,
       Firstly why would you want to run the half wave dipole through the tuner? If its cut correctly for the band in question you should't need to match it to the tranciever.

If the feedpoint is around 50 ohms it doesnt really matter what length the 50 ohm coax is (losses aside) and you should only have minimal mismatch if any.

Just my thought but maybe the tuner cant find a match because there is no transformation needed in the system. ( assuming the dipole is tuned for the band in question)

check what the vswr meter on the radio reads with the atu out of line.

billy uk
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K7MDO
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« Reply #4 on: September 25, 2008, 02:38:49 PM »

I am sorry, it is the feed line that is 18 feet long, not the dipole.... should have made this clear.... Tom
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K7MDO
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« Reply #5 on: September 25, 2008, 02:40:24 PM »

M0JHA, thanks for that input, I will try that... don't know why I was so set on the tuner being involved... will see what happens tonight...  Tom
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W5DXP
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« Reply #6 on: September 25, 2008, 03:39:07 PM »

What is the SWR at 50, 52, and 54 MHz?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
AA4PB
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« Reply #7 on: September 25, 2008, 06:33:07 PM »

A tuner should easily match a 50 ohm load. If the antenna is exactly 50 ohms (which is highly unlikely) then the SWR will be 1:1 with the tuner on or off.



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K0ZN
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« Reply #8 on: September 25, 2008, 06:52:18 PM »

Hi,

Something isn't right.... Properly built, wired and assembled (to correct length) , a 6 M dipole connected to good quality 50 ohm coax should be a close match; maybe not dead 1:1 but certainly a "low" < 2:1  SWR.

Check your length on EACH side of the center insulator
it should be very close to 54.3 inches. (assumes you want a 51 Mhz resonance). I have to assume you are using good quality insulators, etc. You will probably have to do some minor tweaking due to the added capacitance of the end insulator loops and the added length from the coax leads at the feed point, but it should be "In the ball park".

Is the antenna in the clear?!  If the antenna is near a metal object such as a gutter, window, house wiring, metal roof, etc. that could change the feed point a fair amount. Is the transmission line perpendicular to the antenna? If the coax is much closer to one side of the antenna it can goof things up sometimes.

Check your coax connectors are they good, clean and properly wired? If you are using some super cheap coax or coax that is NOT 50 ohms, you may have a problem.
Are the connections between the antenna and the coax good, clean, secure connections?

A dipole is a "flat" antenna with naturally low SWR (if fed with 50 ohm coax) so feed line length is NOT critical or important. Coax length "issues" and myths are something that has escaped from the CB band; it is a non-event when feeding an antenna where there is a good impedance match between the coax impedance and the antenna impedance.

73,  K0ZN
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WB2WIK
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« Reply #9 on: September 25, 2008, 07:38:14 PM »


>RE: 6 meter dipole feed line....    Reply
by K7MDO on September 25, 2008    Mail this to a friend!
I am sorry, it is the feed line that is 18 feet long, not the dipole.... should have made this clear.... Tom<'

::An 18' feedline is way too short.  Here's the reason: That means the dipole is too low!

The goal is to have a very LONG feedline, and make sure most of it is running vertically.

In any case, a 9'2" long dipole requires no tuner, period.  If you're having problems, the tuner itself could be causing them.  Remove it from the circuit, and just transmit into the dipole, it should work just fine.

SWR of a 9'2" long dipole, center fed at 20' above ground, at 51 MHz, is 1.5:1 worst case, and usually better.

WB2WIK/6
 
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K7MDO
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Posts: 325




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« Reply #10 on: September 26, 2008, 07:23:31 AM »

OK, I think the dipole is cut too short or it exhibits a "too short" condition where I have it hanging... admittedly not in the best location.  

The measuered swr at 53.9 Mhz is about 4:1 and at 53 it is way over 10:1 and same at 51 mHz... so it is cut for somewhere north of the band... or the adjoing building is influencing it in that direction....  

I did notice in the ic-756 manual that the internal tuner simply cannot handle out of swr levels above about 2.5:1 so that explains why it can't bering the antenna into any acceptable condition for the radio....

This weekend I am going to extend the length about a foot and then do the same tests.... will report.

73
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M0JHA
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« Reply #11 on: September 28, 2008, 04:08:34 AM »

Try another dipole head hight for ease of trimming /feeding and see how you get on.
You should't have any problems actually tuning a halfwave dipole at this height just for experimenting.

Is your coax ok? if you have a dummy load try this at the dipole end of the feed you should get a low vswr reading if the coax is fine.

if you havn't got a dummy load invest in one they are cheap and also very easy to make one yourself if you wish and are a great tool to have..

billy uk

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K7MDO
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« Reply #12 on: September 29, 2008, 07:35:18 AM »

well, the weekend was pretty productive on this problem.  I carefully retrimmed the the antenna to the correct length.... originally I had ignored the lengths going to the coax connector inside the center insulator... by nipping off 3" on each side of the dipole it certianly worked better.... now the other part of the puzzle... "ignorance", my own.... I have only owned the ic-756 for a little over a year and had never used its internal tuner... and hadn't read the directions very well either.  As it turns out, the tuner is "automatic" and does not function like the external AH-4 that I have used since I got the radio.  I kept trying to force it to tune by pressing and holding the tune button for two seconds... turns out that this is unnecessary.. you simply momentarily press the tune button and that initiates the thing for its "auto" function... as I cruise across the 50 mHz band it cheerfully retunes as I go through the frequencies...  

SWR for that antenna on transmit is almost 1:1 throughout the band... I imagine the internal meter in the Icom is not the best but for now: I am on the air on 6 meters.... it just seems awfully quiet....  thanks to those that responded, 73 Tom
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M0JHA
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« Reply #13 on: September 29, 2008, 08:49:15 AM »

job done.. :-)  billy uk
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