[TenTec] The OCFD Mystique
Rick - DJ0IP / NJ0IP
Rick at DJ0IP.de
Wed Sep 23 05:03:29 EDT 2015
Boy there are a lot of wrong statements in that one, Barry.
There was good sound advice, mixed in with some totally wrong statements.
"So Windom said I have the answer. I'll feed the antenna at a point that would reflect a reasonable impedance regardless of band. THERE NEVER was a consideration for SWR. The SWR issue ONLY came up after coax cable began to be used widely."
THIS IS TOTALLY WRONG! Windom never said anything of the kind!
Windom NEVER set out to create a harmonic antenna with equal impedance on all bands.
His only goal was to find a position to feed the Herz, such that there were NO STANDING WAVES ON THE FEEDLINE! THAT WAS HIS ONLY GOAL!
Read Kruse's 1926 QST article and read the original Windom Paper (1929 QST).
I gave exact details of where these are in my previous email.
"Next, a Windom type antenna, dipole, and OCFD are the exact same thing. The only difference is the feed. A 132' wire will have generally the same gain regardless of how you feed it. However, the pattern will be skewed a bit showing a bit more gain in one direction feeding it off center."
THE ONLY THING THESE ANTENNAS SHARE is approximately the same radiation pattern.
The Windom (the real one) as well as the false Windom which is what most people today believe to be the real Windom, both needed a good ground network to be efficient. The dipole and OCFD do not.
The dipole and OCFD are similar but the OCFD has significant common mode current characteristics that must be addressed.
"Most Windoms, OCFDs are designed for a 300 Ohm feed point making it necessary to use a 6:1 balun transformer."
WRONG: Most OCFD antennas today use 4:1 baluns. You can't get the OCFD to have 300 or 200 Ohms. You might get a single band to match that, but that's it. The impedance is on the order of 230 to 240 Ohms when it is elevated around 30 to 50 ft - and it is about 260 to 280 Ohms when it is above 70 ft. Obvious ground type and other environmental thing influence this.
This point was researched by Buck Rogers (K4ABT) long ago and has been on his web site for as long as he has had one. At one point in time Buck sold 5:1 baluns with his OCFD's but these were complicated beasts and he soon went back to 4:1.
"And if you have RF in the shack, you are not using a good balun or have coupled energy from the antenna to the transmission line, not an uncommon issue if you do not come away from the antenna at a right angles."
BARRY IS ABSOLUTELY RIGHT ON THIS POINT! THIS IS VERY VERY IMPORTANT.
Unfortunately I know of no commercially available balun at this time that will sufficiently deal with the common mode current problems of the OCFD on 80m. As a result you should always use an additional RF choke at the feedpoint. A Maxwell might suffice. A Guanella will.
If anyone thinks (s)he know of one, don't just call it by the manufacturer's name, tell us the make and model number and be prepared to tell us what's inside the box. If you don't know, then how can you know it is good enough?
"Now having gone through all of that, let me say, A very good antenna is a center fed 1/2 wave on 80 meters fed with open wire, if you have a fair run. Where you enter the house/shack, you transition to something like LMR-400 using a good 4:1 current balun."
SORRY, NOT CIGAR. DO NOT (only) USE A 4:1 BALUN in front of your tuner.
The problem here again is the common mode current and it must be removed. A 4:1 balun, regardless of how good it is, is rarely good enough. YOU MUST USE A 1:1 GUANELLA BALUN HERE!
If your matchbox is incapable of matching the 2K+ Ohms on some bands, then it is fine to insert a 4:1 between the 1:1 and the openwire feedline. But it is used "in addition to", NOT "instead of" the 1:1 balun. BUT REMOVE THE 4:1 BEFORE OPERATING ON THE FUNDAMENTAL FREQUENCY OF THE ANTENNA.
This is explained (proven) by W8JI and G3TXQ.
I have proven it too (in practical testing) but I'm not an engineer so my word does not count. ;-)
PEOPLE, FORGET ALL YOU THINK YOU KNOW ABOUT BALUNS & TUNERS, MASTER CLEAR AND DO YOUR OWN DUE DILIGENCE.
(use the links below to the real gurus of the industry)
Balanced Tuners and common mode current, by VK2OD
Tuner Balun Ratios: 4:1 or 1:1? , by G3TXQ
Tuner Baluns, by W8JI
Choosing the Correct Balun, by W8JI
RFI-Ham, by K9YC
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