I would strongly warn everyone who uses this method of feeding the centerfed "tuned feeder" dipole about a problem with this. Let me start by saying I have done it and still do it with lowered powered rigs (under 100 watts). There IS a danger, that of overheating the 4:1 balun and losses in the balun ( or even destruction!) under some conditions. Most "baluns" want to see a relatively reactance free impedance transformation. Also one which is close to the transformation ratio (commonly 1:1 or 4:1) I use a 135' dipole at 50' with about 100 feet of 450 ohm line. The impedances presented to the "generator" are usually not (in my case) a 4:1 ratio or have a lot of inductive or capacitive reactance on some bands. Broadband baluns don't like this condition and when higher powers are used,the baluns will cause losses and/or overheat under these conditions. After playing with baluns for quite a number of years, I decided to try the old fashioned "balanced line" antenna tuner configuration instead of the "T" network high pass transmatch (most commonly available "tuner") and a 4:1 or a 1:1 broadband balun. Yes, the balun WILL work under a lot of conditions but not really well. Only if the transformation ratio is "in the ballpark" or a high reactance condition does not exist and the feed point presented to the output end of the balun. A "balun" is no cure all or "simple solution" for a proper matching network, or the "proper network" for matching an unbalanced 50 ohm transmitter to a "tuned feeder" type dipole. I repeat: It WILL work or appear to work under a lot of circumstances, but often times is an inefficient way of doing things. One quickly finds out about this when using higher powers and when the conditions are not within the normal parameters of the "balun". I repeat again, I have done this many times, but I usually cringe at doing it because of the reasons stated. If you are running QRP, you want every microwatt you can muster into the "ether" to be radiated, not thrown away as heat. Ditto for high powers, except at kilowatt levels, sometime improper devices will burn and smoke or at least get very hot. Why throw the power away?
My two penny's worth. 73, Sandy W5TVW ----- Original Message ----- From: "George" <[EMAIL PROTECTED]> To: <elecraft@mailman.qth.net> Sent: Tuesday, October 03, 2006 3:23 PM Subject: [Elecraft] 135 ft flat top antennas, again, ad infinitum | My 2 cents worth is: Theory is one thing and actuality is another thing. I use a 130 ft inverted vee, fed with about 55 ft of 450 ohm ladder line. The apex of the vee is about 65 ft up. The ladder line stops at the eave of my house, just outside the shack, where I use a 4:1 balun. >From the balun to the homebrew tuner, I use a 7 or 8 ft piece of RG 213. It >works FB on all bands from 80-10 m, including 30 m, and is absolutely the best all band wire antenna I have used in my 51 yrs of hamming. It works especially well on 75 and 80 m. It would possibly work even better if I broke up (with insulators) the guy wire which is almost parallel to one leg of the vee. Also, the antenna is surrounded by several trees which are 15-25 ft higher than the antenna. I guess the moral to this is, don't let theory stop you from trying stuff. | | 73, N4ym | _______________________________________________ | Elecraft mailing list | Post to: Elecraft@mailman.qth.net | 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 | | | | -- | No virus found in this incoming message. | Checked by AVG Free Edition. | Version: 7.1.407 / Virus Database: 268.12.12/461 - Release Date: 10/2/2006 | | _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
