In a message dated 5/17/07 11:47:30 AM Eastern Daylight Time, [EMAIL PROTECTED] writes:
> The 5RV will only provide the correct match transformation at a specific > height and in a straight line. If the ends droop the impedances are > wrong and the match between the balanced line and the coax will be > wrong. > > That's true in theory. But how "wrong" it is in practice varies all over the place. And it's true for any dipole, not just a G5RV. Depending on how far from the ideal the system varies, the additional mismatch due to droop and such may not be worth worrying about. Any tuning unit should therefore be between the coax and the > > balanced line. Tuning the coax is the shack is bad practice and is > likely to be lossy. It's not that simple. If the SWR on the coax line is high, the coax is small, and the line is long, the result can be a lot of loss. But if the SWR is reasonable (say, less than 3:1) and the coax is relatively short and low-loss (say, 50 feet of good quality RG-213), the resulting overall system loss will probably not be worth worrying about. The trick is to calculate just how the system will *actually* work. With computer models like EZNEC and Reg Edwards' DIPOLE3, the effects of things like SWR on the coax can be found with a reasonable degree of accuracy. A much better alternative is the doublet fed with > > balanced line all the way back to the shack. That's only true *IF*: - the balanced line can be run into the shack without undue mechanical problems - the tuner used to match the balanced line is up to the job on all frequencies of interest - the overall resulting loss from all sources is kept low. Those "ifs" are not guaranteed in all cases. For example, consider a 130 foot dipole fed with 60 feet of open wire line. Compute the impedance at the shack end of the line on the various bands, and see how well the various common tuners can match it. Forget the 5RV and get as > > much wire as you can in the sky. All the G5RV amounts to is a way to feed a wire of about 31 meters so that the matching job is made easier. The same is true for OCF dipoles. Depending on the individual situation, a G5RV may be the best solution - or an OCF, or the classic dipole-fed-with-ladder-line. Trap dipoles are also lossy. Again, that's too general a statement. Yes, a trap dipole made with several traps that have low Q will be less efficient than a full size antenna. But a trap dipole with only a few traps of lowloss construction can be within a decibel of a full-size antenna - on multiple bands - and can often fit where a full-size antenna will not. In practice, less than a decibel of loss usually isn't worth worrying about. > A good read on antenna theory can be obtained from the RSGB bookshop and > HF Antennas for All Locations by Les Moxon G6XN > There's also W4RNL's excellent website. But most of all, do not simply dismiss whole classes of antenna because some versions can behave poorly. 73 de Jim, N2EY ************************************** See what's free at http://www.aol.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
