I have used a doublet fed with open wire line most of my half century on the air, and what Don says agrees with my experience.
I have measured the current in each leg of the feed line with an unbalanced tuner and no balun and found it very well balanced. The balance in the currents seems to have a lot more to do with the load (antenna) than the source. After all, the RF currents coming out of a BNC or other coax connector are 180 degree out of phase, just like the output of the best balun or balanced tuner. The only difference is that in a normal coax output one side becomes coupled to the rig case without a contiguous coaxial shield to prevent it. How much that affects the balance depends entirely on the impedance of the feedline at that point, the size and capacitance of the rig to ground and the impedance of any other ground paths present. Lots of variables! As Don said, a choke (current) balun can do wonders if you have serious unbalance or "RF" on the rig. It removes the common-mode (in phase) currents flowing on the line without upsetting the RF 180 degrees out of phase moving from the rig to the antenna. That effectively prevents the RF flowing up the open wire line from flowing back along the outside of the rig. A doublet will work very well at lengths down to 1/4 wave overall, as long as your tuner can match the impedance. So a 66 foot wire will do a very good job on 80, or a 135 foot wire will do a very good job on 160. The difference between such as "half sized" doublet and a full 1/2 wave long doublet is about 1 dB or so, from what I've read. The issue on the lower frequencies with a horizontal doublet is more a matter of antenna height than anything else. Below 1/2 wavelength high, the low angle radiation starts to suffer and by the time the antenna is 1/4 wavelength above the ground the main lobe is straight up! It can have considerable gain at 0.15 to 0.25 wavelengths high -- something on the order of 6 db gain -- but it's straight up. That's excellent for NVIS (short skip) QSOs out to 500 miles or so, but lousy for DX. On 20 meters, you'll have excellent low angle radiation as long as the wire is 30 feet or so up. Of course, hanging the doublet as an inverted V or as a sloper will add some vertical radiation to the mix that will help the low-angle field a bit. If you can't get a match and put a 1:1 balun in the system and the ATU matches it, it's most likely because of losses in the balun! The balun should report the same impedance to the tuner as the antenna did without it. If the system matches with the balun but won't without it, the balun must be injecting some impedance, and that impedance will be a loss impedance. If you try a 4:1 balun and get a match, that may be because of the impedance transformation occurring in the balun, or it may be loss. It's hard to tell unless you discover your balun core is noticeably warm. Large baluns dissipate heat so well it's hard to tell. They aren't necessarily any more efficient. They simply disguise the loss more effectively with a larger core mass to absorb the heat produced by the losses. The bottom line is that transformer baluns get unpredictable under high SWR conditions. One may work well under those conditions or it may not. There's no easy way to tell. That's why I avoid baluns on an open wire line unless I really have to use one, then I'll look to a current balun as Don recommended. If my tuner won't provide a match on one or more bands, changing the antenna length or feeder length is the best bet. It's a matter or cut and try, but it results in the most efficient system under the conditions. Ron AC7AC -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim Miller Sent: Tuesday, July 31, 2007 7:36 PM To: [email protected] Subject: Fw: [Elecraft] K2 Tuner? Don/Leigh Thanks for the quick replies. I'll try the K2/KAT2 nekkid for this Thursday's 20m fox hunt unless I can adapt one of my ferrite bead coax chokes by then. I normally use a remote tuner but it has died again, this time outside the warrantee period. I plan to return it to see what they say but I'm not optimistic. I'm going to try using a T1 as my remote tuner in any case. As a first try I'll just connect my feedline from the antenna to a BNC-bindingpost adapter to see how it tunes. I can insert a BL2 which I've ordered set at 1:1. I'll also add a ferrite bead coax choke for good measure to reduce any common mode currents on the coax from the shack to the tuner. I'll probably try the ferrite bead choke first. 73 jim ab3cv _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
