Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
Questions on broadband antenna design (e.g. T2FD)
In article ,
C. J. Clegg wrote: Actually, what I mean is that some, including me, will be using 5-watt radios (Yaesu FT-817). If the option of pouring 100W into any hank of wire was available to all, the issue wouldn't be critical. It's not, so it is. :-) There's another approach to the antenna that you might want to consider, _if_ the ALE ranges are limited to a few (say, no more than four or five) and aren't too wide (say, no more than perhaps 5% of the center frequency). If your usage pattern fits this model, then you could construct a multi-wire "fan" dipole array... simply a set of individual wire half-wavelength dipoles, each cut for the center frequency of a given ALE range, spread apart physically, and fed from a single coax at a single feedpoint. They can be spread vertically (e.g. hang the longest one as a flat-top from a convenient set of trees, suspend the next-shortest beneath it on 6" spreaders, hang the next-shortest on another set of spreaders, etc.) or horizontally (run one north/south, another east/west, etc.). There will be some amount of coupling/loading between them, so you'll need to trim them for lowest SWR once they're installed, and you may find the SWR bandwidth less than you'd get from individual dipoles. Coupling/loading is greater for the vertically-stacked case, and less for a star-like horizontal pattern (which takes more space and more trees or etc., of course). The nice thing about this approach, if it's suitable for your needs, is that the losses are quite low. The wire which is resonant on the band in question loads up and radiates, and the other wires (which will typically have high, reactive impedances) accept very little current. No lossy termination resistor is needed. I've been using a three-wire fan dipole (5" vertical spacing) cut for 40/20/10 for several years, with very satisfactory results. This approach won't give you continuous coverage of your 4:9 range... unfortunately I can't suggest any which would, which don't require an agile tuner/transmatch and which don't introduce high losses in some portions of the band. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Starting point for antenna design | Antenna | |||
Eton Porsche Design P7131 Questions | Shortwave | |||
The main problem with Ham radio... | Policy | |||
BiQuad Design Specifications for Microwave? | Antenna | |||
Response to "21st Century" Part Two (Communicator License) | Policy |