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Message |
    
Joey_migs
Junior Member
Username: Joey_migs
Post Number: 21
Registered: 9-2009
| | Posted on Sunday, September 27, 2009 - 4:35 pm: | 
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I am trying to construct a 11 meter dipole.
I made a 1.5:1 transformer (to better match the 72 ohms dipole feed to the 50 ohms transceiver)using a FT-240-61 ferrite toroid, a trifilar winding of no. 14 enamel coated copper wire and a couple of capacitors. I tested the setup using a RG-8 to transformer to RG-59 terminated with a 72 ohm dummy load - the SWR was less than 1.3:1 across the band.
My question is the dipole is balanced and the coax is unbalanced will this cause any problems?
Joe |
Tech237
Moderator
Username: Tech237
Post Number: 1228
Registered: 4-2004
| | Posted on Sunday, September 27, 2009 - 7:18 pm: |
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What you really need , if you both to use anything, is a balun. Personally I'd use a 1:1 balun as the minor mismatch is no problems.
The other option is not to use anything and wind SWR around 1.5:1 across the band.
Either the balun or the choke takes care of the unbalanced to balanced issue.
Out of curiosity what power did you test your transformer (actually if you used capacitors you reall have a two close coupled tuned circuits) adn what was the power on the 72 Ohm side?
Simon
Tech237
N7AUS
.
I thought he said, "there was no rust for the wicked, and I own an MGB"
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Bruce
Senior Member
Username: Bruce
Post Number: 5186
Registered: 9-2003
| | Posted on Sunday, September 27, 2009 - 7:02 pm: |
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should work |
Tech833
Moderator
Username: Tech833
Post Number: 1750
Registered: 8-2002
| | Posted on Sunday, September 27, 2009 - 8:39 pm: |
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You had me right along with you until you mentioned 'couple of capacitors'. An impedance transformer won't work properly over a bandwidth unless it is a low-Q circuit.
The bandwidth will be always be very narrow if you are using capacitors as a means to balance the network. Also (I am sure you already know this) the power capability will be greatly limited by your ferrite material. The ferrite in the high-Q will saturate at some point and be ineffective as power level increases (like on modulation peaks). Your SWR might show good bandwidth, but at the band edges, power is merely being absorbed, not transferred.
A standard balun is certainly the best bet here for maximum power transfer and optimum bandwidth. A 1:1 as Tech 237 mentioned is a solid choice as he is correct that the small mismatch will be inconsequential.
One of my 10m dipoles is a wire type fed with a 1:1 balun and it covers the entire 10m band under 1.2:1 SWR and even covers 11m at around 1.5:1 in the middle of the band. That bandwidth is REAL bandwidth as there are no tuning components in the balun, all power is transferred from the feedline to the wire elements.
And, they perform quite well if you can get them more than 1/2 wave above ground.
No matter what, have fun with your project! Experimentation is the key to learning. |
Joey_migs
Junior Member
Username: Joey_migs
Post Number: 22
Registered: 9-2009
| | Posted on Sunday, September 27, 2009 - 11:59 pm: |
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I tested using 40 watts in 50 ohm side and I got slightly less then 40 watts on the 75 ohm end. I was concerned with core saturation, as well. I wish I had a spectrum analyzer to check for spurious signals across the frequency range but unfortunately I don't. I do have a temperature probe for my Fluke so I used it to check if there was a significant temperature increase in the windings during key down from band-end to band-end. There was no significant increase of the winding temp. This may be due to using relatively low power and I may have to watch out if I decide to increase it.
I am going to work on a building 1:1 Balun, but I have to research it a bit first.
Thanks to every one who responded, it lets me know I am moving in the right direction,
Joe |
