[Antennas] Re: T2FD

[fkamp at comcast.net]

Martin Ewing wrote:
> 
> A few comments on the terminated folded dipole,
> called T2FD when tilted.  I have been using the 90 ft
> B&W version for several years.
> 
> First, I think the Cebik analysis at
> http://www.cebik.com/wbfd.html is very useful.  He
> also has an earlier note on the T2FD version at
> http://cebik.com/t2fd.html.
> 
> The takeaway for me is that the antenna begins to fall
> apart at 4 MHz as you lower the frequency.  Above 4
> MHz, you "only" have a loss of 3-6 dB.  B&W has
> marketed this antenna for 160 M and up.  That's a big
> stretch!
> 
> The big benefit is that you get "useful" performance
> from 40 - 10 M and "some" performance at 80 M -- all
> in one antenna at a manageable SWR.
> 
> So what about that 6 dB loss?  That's 1 S-Unit.  How
> many QSOs are impossible if you cut your power by 1
> S-Unit?  Some, but not most - IMO.  On the other hand,
> 6 dB means that 1/4 of your power goes into the air,
> while 3/4 goes into the terminator!  No good for QRO.

It is even worse for QRP.  You can overcome the loss on transmit by
increasing transmitter power, but there is little you can do to ballance
out the loss on receive.

> 
> I recall the QST article a few years back about using
> a lightbulb as a broadband antenna.  The good news is
> that the terminated folded dipole is a lot better than
> a lightbulb, except maybe on 160 M. (On the other
> hand, you can read a newspaper with the lightbulb. :-)
> 
> I recently put up a 3-el SteppIR at 40 ft, and the
> world came alive on 20 M.  I gained about 2 S-Units on
> receive, and I started getting compliments on a big
> signal in Europe (with 100 W).
> 
> I have to admit that one reason I put up the folded
> dipole in the first place is that it _looks_ neat.
> Like a 1930's installation... The main reason was that
> I could get all bands, including WARC, with just one
> installation.  And I'm a closet QRPer.
> 
> But it feels so good when you put up an efficient
> antenna.
>

It feels even better if you can do it for under $200.  Last time I
checked the B&W antenna was priced at $200.  I am sure it is of
exceptional quality, but I can build a very nice antenna system for
$200.
 
My only real experience with broadband antennas is with a VHF discone. 
It did not take long to discover that such an antenna was not very good
for 2 meter FM.  Too many unwanted, strong, out-of-band signals.  Of
course that is what you get with a broadband antenna.  The advantage of
having a low SWR over a range of frequencies is more than offset by the
unwanted broadband response on receive.

In my opinion, the only GOOD broadband antenna for any range of
frequencies is the log periodic.

Regards,
Frank Kamp
K5DKZ