Here is W8JI's take on the noted subject:
http://www.w8ji.com/short_dipoles_and_problems.htm
Some key points:
"While pattern looks excellent on higher bands, lowest band performance
of a short dipole (in this case 88 feet long on 80 meters or 44 feet
long on 40) certainly leaves a great deal to be desired when feedline
and tuner losses are included. It isn't the efficiency of the antenna
that is in question, it is feedline efficiency and high voltages in the
matching system that become problems.
"Short dipoles and verticals will almost always model with very good
efficiency, because a model is a perfect lossless system if we do not
include feedline losses and matching losses.
".... (It makes very little difference by the way if the balun is on the
tuner input. Don't think moving a current or choke balun to the input of
a tuner is a solution. The balun is under the same common mode
stresses.)
"The lower limit in size of a multiband dipole, before feed system and
matching losses move to the edge of severe problems, is about 200 feet
on 160 meters, 100 feet on 80 meters, 50 feet on 40 meters, and so on.
"A good rule of thumb is to multiply the band in meters by 1.25. The
result is the minimum dipole length you can use without using a good
matching system in, at, or near the antenna!
"As an antenna is shortened from that length, losses in the feed system
(even what Hams consider a good one) climb rapidly. 88 feet is just too
short for an 80 meters antenna, because...it is at the limit of what
most tuners will match. It also places most tuners at their power limit
at a few hundred watts of applied power.
(Take heed of the above paragraph if you are running HP! --Matt, K4AQ)
"The optimum length for a multiband dipole is near 1/2 wl on the LOWEST
band, and the optimum open-wire feedline length is any odd multiple of
1/8th wavelength on the lowest band. This means an optimum 80-meter
dipole would be about 125ft long, and the feedline would be 25-30ft,
75-90ft, or 125-150ft long. The longer the feeder, the more likely you
are to having to trim it for optimum tuner performance.
"Most tuners like to see impedances HIGHER than 50 ohms, and inductive
loads at low impedances. Pi's and L's are NOT a solution to matching
problems. They actually are significantly more restricted in matching
range than a conventional T using the same general style and quality
components."
--
Matt Lee, K4AQ
Atlanta, GA USA
<K4AQ@arrl.net>
> -----Original Message-----
> From: secc-bounces@contesting.com
> [mailto:secc-bounces@contesting.com] On Behalf Of ku8e@bellsouth.net
> Sent: Monday, 08 November, 2004 11:40
> To: Tommy; SECC@contesting.com
> Subject: Re: Re: [SECC] K4OGG, CWSS
>
>
> Tom,
>
> If you are using a 130 foot antenna with open wire feeder....
>
> On 80 meters it acts like a dipole
>
> On 40 meters if is two half waves in phase broadside to the
> antenna giving you about 3 db gain over a dipole
>
> On 20 meters you have a clover leaf pattern.
>
> On 15 meters you have about 8 lobes
>
> On 10 meters you have a whole bunch of lobes !!!
>
>
> If you can only put up one antenna this is the one. you
> could short the feeders together and run it as a "T" on 160
> and put down some radials. You should get out good enough
> (running the amp) to be dangerous !!!
>
> See http://www.cebik.com/88.html
>
>
> 73, Jeff
>
>
> >
> > From: "Tommy" <aldermant@alltel.net>
> > Date: 2004/11/08 Mon AM 10:37:44 EST
> > To: <SECC@contesting.com>
> > Subject: Re: [SECC] K4OGG, CWSS
> >
> > My Titan 3 blew up, sitting in stand-by on the desk. Just got it
> > back from Ten Tec on Thursday.
> >
> > When testing it, could only get 1200w max into the dummy load.
> > Taking it apart and inspecting it, I found the same old problem of
> > poor assembly. A screw from the tube section was protruding to the
> > below compartment, partially shorting out the RF input line on a PC
> > board. I went through this with Ten Tec back in November and found
> > the problem after they told me "you must have a bad interface coax
> > cable". Still an on-going problem with practically NO QC at Ten Tec.
> >
> > Anyway after replacing the screw, I still expected to have problems
> > with the amp, but it just sat there, purring at 1500w.
> >
> > Was going to add an HF (20&15m) antenna, but the way this silly 80m
> > dipole is working...WHY?
> >
> >
> > Tom - W4BQF
> >
> > ----- Original Message -----
> > From: "Macie, Gordon" <GMacie@innotrac.com>
> > To: "'Jay Pryor'" <jpryor@uga.edu>; <SECC@contesting.com>
> > Sent: Monday, November 08, 2004 10:13 AM
> > Subject: RE: [SECC] K4OGG, CWSS
> >
> >
> > > I ran my amp also. Strange experience people actually hearing me.
> > > I had very few fills compared to what I'm use to.
> > >
> > > Gordon
> > >
> > > -----Original Message-----
> > > From: Jay Pryor [mailto:jpryor@uga.edu]
> > > Sent: Monday, November 08, 2004 10:05 AM
> > > To: SECC@contesting.com
> > > Subject: [SECC] K4OGG, CWSS
> > >
> > >
> > > This was my first part-time CWSS in a long time. And the first
> > > time I have
> > > ever entered any contest in the high power category. But there's
> > > no question that my score is higher than it would have been
> > > otherwise. Anyway, it took me a while to figure out how to hook
> > > up and
> > > load up the amp. I really don't use it much. After I got it up
> > > and
> > > running though I enjoyed a laid-back SS. It was good to run into
> > > many SECC folks, and other contesters who are "regulars."
> > >
> > > Here's what it looked like:
> > >
> > > BAND Raw QSOs Valid QSOs Points Mults
> > > __________________________________________________
> > >
> > > 80CW 64 64 128 30
> > > 40CW 48 48 96 18
> > > 20CW 103 103 206 14
> > > 15CW 95 95 190 9
> > > 10CW 12 12 24 1
> > > __________________________________________________
> > >
> > > Totals 322 322 644 72
> > >
> > >
> > > Final Score = 46368 points.
> > >
> > > 73,
> > >
> > > Jay/K4OGG
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