Jim was asking about feeding an unbalanced antenna (vertical), not a
dipole, however your idea still applies.
Actually, an air core transformer would work (almost). Taking this
thought and evolving the network...
If the half wave antenna is resonant, it is all resistive, and by
putting a transformer winding in series with it, which has significant
leakage inductance, you also have to put in a cap in to cancel the
inductance that the antenna sees. Now you have the equivalent of a
transformer plus parallel LC network. For simplification, you can just
leave out the seperate primary winding and tap the remaining winding to
connect the feedline. Now you are back to the textbook way of feeding
these things.
And yes, a narrow band network would work actually work better, given
the difficulty of building a broadband high impedance transformer.
If you used this on a balanced antenna such as a dipole, you would need
a choke of some kind to eliminate common mode currents (assuming you
care), but that is another topic, (although an interesting one).
Jerry, K4SAV
Jim Lux wrote:
>At 11:11 AM 6/1/2005, K4SAV wrote:
>
>
>
>>I agree with Tom's assessment of feeding a half wave antenna. After
>>thinking about using a transformer, there is one parameter which is
>>really difficult to overcome, stray capacitance between primary and
>>secondary. Since this transformer is connected to the high impedance
>>end of the antenna, it only takes a few pf and you don't have much of a
>>transformer. For example, if the antenna impedance is 4000 ohms, an
>>assuming a 10MHz antenna, a primary to secondary capacitance of 4 pf
>>produces about 4000 ohm load (reactive). Since this transformer has to
>>have several turns, it is going to very difficult to keep the
>>capacitance low. The transformer also has to be large, with lots of
>>ferrite material to keep the loss in the ferrite low, which further
>>complicates the capacitance problem.
>>
>>
>
>
>Is there a particular reason why one would want a broadband transformer
>here? A dipole is going to be a fairly narrow band device, so why not use
>a narrow band matching network of some sort.
>
>Also, why use a cored transformer instead of going air core? Getting a
>100:1 impedance ratio is only a 10:1 turns ratio. You'd actually need to
>fiddle around a bit, because air core transformers don't have as much
>coupling between turns, so there'd be some leakage inductance to deal with.
>
>In any event, some sort of tapped air core inductor might be the way to
>feed the end of this thing.
>
>
>
>
>
>>This transformer also would not be useful on low impedance loads. You
>>would have to switch it in and out.
>>
>>I gave you an example from ON4UN's book, that I remembered, without
>>thinking much about the technical merit or problems associated with
>>this. It's going to be much harder, trying to make this transformer
>>work well, than just using an LC network.
>>
>>Jerry K4SAV
>>
>>Tom Rauch wrote:
>>
>>
>>
>>>I can't think of a more difficult thing to do *properly*
>>>than to end feed a half wave. There are a good may
>>>half-baked feed systems around, but the only reliable and
>>>simple system is a resonant tuning unit at the base of the
>>>vertical with the system worked against a ground system. A
>>>stub would be a good second choice, but a single band affair
>>>and you would be faced with some radiation from the stub.
>>>
>>>A much better solution for multiple bands would be a trap
>>>vertical mounted as high as possible worked against a ground
>>>system of drooping radials.
>>>
>>>A thick element could have an impedance as low as several
>>>hundred ohms, a very thin half wave element maybe 3000-4000
>>>ohms. The problem is the reactance off frequency, and how to
>>>compensate. It will never be multiband without traps or
>>>complex stub decoupling systems on the element.
>>>
>>>Trying to use a decoupling sleeve is almost certainly a
>>>waste of time. The sleeve ID has to be substantially larger
>>>than the internal coax's outside. Folding braid back is a
>>>common method at HF, but the decoupling performance is
>>>horrible. It's almost like no choke at all.
>>>
>>>http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
>>>
>>>Good luck also on building a broadband transformer that
>>>would match 50 to 4000 ohms. That's another almost
>>>impossible task.
>>>
>>>If you can put a tuner at the vertical base and work the
>>>vertical against a modest ground, that's the best solution.
>>>An elevated trap vertical worked against multiple drooped
>>>radials is the easiest, most reliable, and best performing
>>>solution if you don't want to use a tuner at the vertical
>>>base and want multiple bands.
>>>
>>>73 Tom
>>>
>>>_______________________________________________
>>>
>>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>>>
>>>
>>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>>any questions and ask for Sherman, W2FLA.
>>
>>
>>>_______________________________________________
>>>TowerTalk mailing list
>>>TowerTalk@contesting.com
>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>>
>>>
>>>
>>>
>>_______________________________________________
>>
>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>>any questions and ask for Sherman, W2FLA.
>>
>>_______________________________________________
>>TowerTalk mailing list
>>TowerTalk@contesting.com
>>http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
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