Skipp I sure would be interested in what your version of a beta matched gnd plane antenna which results in a 50 Ohm match looks like. Could you, would you, provide us with a description or picture to satisfy my curiosity? Thanks, a.
--- On Mon, 2/22/10, skipp025 <[email protected]> wrote: From: skipp025 <[email protected]> Subject: [Repeater-Builder] Re: Ground Plane and Dipole Diatribe To: [email protected] Date: Monday, February 22, 2010, 3:41 PM It's Bacon! > That's my point. If there was a specific reason for > making one side of the dipole exactly 5% longer (based on > some pre-determination such as a computer model), then fine. I was talking about 1/4 wave ground plane antennas with 90 degree horizontal radiators... not dipoles, but lets' move the topic forward. The base feed-point impedance of a dipole is not 50 ohms. If you shorten the "hot side" of the dipole the impedance changes and becomes (more) reactive. A beta or equivalent matching network is used to then cancel the reactance and leave a near 50 ohm (non-reactive) base impedance (at the feed point). The matching network is also a much desired DC ground. > But to generically say that all dipoles need to be cut > with one side 5% longer than the other carries no more > engineering support than saying that's how you should > cut the radials on all ground-plane antennas. I'm not even sure where the change from ground plane to dipole started in this thread. I didn't see anyone post all dipoles or all ground planes need to be cut. Just like all ground planes need or should have their horizontal radiators placed at 45 or 90 degrees. Depends on what camp you're in and how close a match you desire to a true non reactive 50 ohm base point impedance. > I wasn't saying that there's never a reason to un-balance a > balanced antenna. I was saying that a generic rule like > "make one side 5% longer in all cases" lacks merit. Yes it does... lack merit. > Kinda along the same lines as "always make the cable from > the connector on the transmitter to the connector on the > duplexer an even half-wave". Most people might/should say it doesn't mater... but in another whole thread we could debate why I think the feed-line length can and in a small way does mater. > As far as an elevated ground-plane with a small number of > radials, it does, in fact, behave more like a dipole than > a ground-mounted ("earthed") antenna, which is why I used > the dipole as a simile in my rhetorical question. Behave more like a dipole... but is not exactly the same animal. > > Because a quarter wave ground plane with a reversed 5% > > longer vertical element/radiator doesn't work well with a > > beta match. > Don't necessarily need a beta match for a groundplane. It > can be regular series-fed, gamma, shunt, whatever. Yeah, but a beta matched 1/4 ground plane is simple and pretty much a true 50 ohm base point impedance with a much desired DC Ground Return. > > I'll email you direct... > > cheers, > > skipp ... and I did. > I'll be on pins and needles! :-) And we made a nice turtle neck sweater to sell at Dayton. not! > Later. > --- Jeff WN3A s.
