Jim.
What is the "right choice" for an impedance transformer core (for a 4;1, 9:1
etc. )? What mix?
Chris
KF7P
On Jun 3, 2013, at 7:59 AM, Jim Lux wrote:
> On 6/3/13 6:32 AM, John Geiger (AF5CC) wrote:
>> I had previously said that I had a G5RV type dipole that I made
>> myself. After repairing it today following a run in with a tree
>> branch last Wednesday, I realize that it isn't quite that. It is
>> more of an "off center fed dipole". It isn't the classic "Windom off
>> center fed" where one leg is twice as long as the onther. On my
>> antenna one leg is approximately 50 feet long, and the other leg is
>> around 60 to 65 feet. It is fed with 300 ohm radio shack twin
>> lead-about a 25 foot run, which goes to a 4:1 voltage balun, and then
>> to 75ohm RG-11 coax to the shack.
>>
>> Given this new revelation, that it is a off center fed dipole, what
>> is now the best balun for the twin lead to coax junction? I am
>> assuming that it isn't quite a balanced antenna, since the legs
>> aren't equal, so how crucial does a balanced to unbalanced (coax
>> feedline) transformer become? My main interest is trying to keep the
>> RF off of the coax shield, although I haven't had any real problems
>> with RFI in the shack.
>>
>
> You've got two separate issues going on here: one is that an OCF dipole has a
> feedpoint impedance that's higher than a center fed. So you probably want
> some sort of transformer. You'd have to model your specific lengths and
> feedpoint position at the frequencies you operate at to find what ratio you
> want. 4:1 and 9:1 are common, because the first is a 2:1 turns ratio
> transformer, the second is 3 turns on the High Z side and 1 turn on the Low Z
> side.
>
> Do you have a way to measure the antenna Z?
>
> The other is that the asymmetric antenna will tend to induce currents on the
> ouside of the feedline (or common mode currents on a balanced line). As a
> practical matter, most symmetric dipoles have this problem too: rare is the
> installation in the middle of a perfectly open field for 100s of meters on
> every side with the feedline perfectly symmetric to the dipole elements on a
> windless day.
>
> The common mode/outside the coax current issue has an easy fix: choke it by
> making a lossy inductor with the right cores (31 mix is favored for this
> application).
>
>
> you do NOT want to use the same kind of core for the transformer as for the
> RF choke. The RF choke core should be one that shows a lot of loss: the
> exact wrong choice for the transformer core.
>
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