[Antennas] RE: B&W Ant. Quest.

Mon Jun 28 22:18:46 EDT 2004

The B&W "wideband" antenna is a take off on the "T2FD" (Tilted
Terminated Folded Dipole) first described by Countryman (USN) in a 1940s
publication! The "secret" to the wide/low SWR bandwidth is the "trap" is
not a trap at all but a 800-900 ohm NON inductive 100w resistor! Noone
is quite sure how he came up with the dimensions he used though and the
"theory" of antenna is still cloudy. The balun used in the antenna is a
16:1 (as measured years ago by myself and KE5O using a commercial
analyzer..it showed with a 50ohm load on the coaxial side, the balanced
side of the balun showed to be exhibiting an 800 ohm impedance)....thus
the Z of the antenna can swing from 400 to 1600 ohms and still have ~
2:1 SWR with the resistor balanced the swing out somewhat!! 
BUT there is a catch....below 10 MHz, the B&W antenna is 6-10 db WORSE
in signal strength than a normal dipole on the same freq (it gets less
pronounced above 10 MHz) ....a dipole fed with ladder or similar line
and a tuner (auto or manual) would produce stronger signals and just as
wide a bandwidth! The received noise is lower on the B&W due to it being
a "grounded" antenna (the center eventually making its way back to the
shield of the coax through the resistor) but then so is the
signal!!...above 10MHz the pattern starts to exhibit some gain with
multiple lobes present but then you get deep NULLS also as you would on
any wire antenna more than 1.5wl long.
For rcv only, just put up a dipole OR horizontal loop at the lowest freq
(or as big as you can get it!). The horizontal loop will give you a
quieter rcvr (due to DC grounding)...don't use the B&W for this...you
will be disappointed (for applications requiring ease of use and quick
install/tear down such as military or utility use on HF by non technical
people, the B&W works ok...but for fixed use, there are better
alternatives)
Chris
WB5ITT
> -----Original Message-----
> From: antennas-bounces at mailman.qth.net 
> [mailto:antennas-bounces at mailman.qth.net] On Behalf Of Orrin Bentz
> Sent: Monday, June 28, 2004 6:18 PM
> To: antennas at mailman.qth.net
> Subject: [Antennas] RE: B&W Ant. Quest.
>>> Hello
>> There is an antenna made by Barker Williamson a model BWD-90.
> This is a folded dipole with spacers and a Balun at the feed point for
> coax.
>> Opposite the feed point there seems to be some sort of trap 
> enclosed in
> PVC tubing. Does anyone have any idea what frequency this trap
> might resonate at? The antenna is 90' long and is touted as being
> 'broadbanded' Even a close guess would be adequate so I can start
> modeling this type of antenna.
>> This antenna is to be used for receive only. www.bwantennas.com
>> Thank You
>> Orrin Bentz