[Top] [All Lists]

Re: [TowerTalk] New Member - Need help with an antenna design

To: "Tim Holmes" <>, <>
Subject: Re: [TowerTalk] New Member - Need help with an antenna design
From: "Jim Lux" <>
Date: Fri, 22 Oct 2004 07:30:37 -0700
List-post: <>
Unfortunately, it is up on the roof now (better than 5 feet off the ground
on the fence).. But.. they're wound on some sort of ceramic forms about 4-5
inches long and 2" in diameter. The winding is something like AWG 20 and
closewound about 3" long. (maybe 60-100 turns?)  I suspect that one could
get the Wheeler (for solenoid inductance) and Medhurst (for self C)
equations and fool around until you got the right L and C for the trap.

There's some articles on the web about making coiled coax traps with design
equations that might be useful.

I have to say that the antenna isn't all that wonderful on 75/80, but that's
more a function of it's location (close to the ground and houses) than the
inherent design.

Overall, the antenna is about 80 feet long.

----- Original Message -----
From: "Tim Holmes" <>
To: "Jim Lux" <>; <>
Sent: Friday, October 22, 2004 3:56 AM
Subject: Re: [TowerTalk] New Member - Need help with an antenna design

> Jim:
> That Alpha Delta Antenna that you described sounds really interesting,
> Do you have any ideas how to construct the loading coils for the 80
> meter segment?    The AT11MP tuner that I have really does a great job,
> and it will handle a 10:1 mismatch without too much problem, so I'm not
> gonna be obsessive over the match, I just wanna get it close so I can
> put some RF Into the air
> Jim Lux wrote:
> >Since you have a tuner, you're in great shape.  The usual problem with
> >band dipoles is trying to get a decent match on all bands, because the
> >all interact with each other. For you, though, it's probably ok if the
> >perfect match for 20m is at 13.9 MHz or 14.4 MHz, because the tuner will
> >it.
> >
> >A couple ideas.. I have a Alpha-Delta DX-CC which is a multiband dipole
> >3 elements. A 10m, a 20m, and a 40m (which serves on 15 as well, by
> >operating as 3/2 wavelength).  It has a funky loading coil about 10 feet
> >from the end which does the 80m band.  The 3 wires are supported about 6"
> >apart by plastic spacers (1/2" PVC pipe would work well, I think,
> >if you got the pink UV stabilized kind).  Originally (before tuner days),
> >tuned it up using a MFJ antenna analyzer, and it was a bit tedious.
> >with the 10m element, adjust, then adjust the 20 element length, then
> >the 40 (inside the coils, then adjust the tails for 80, repeat....
> >
> >Dipoles are fairly broad band (typical 2:1 bandwidth is on the order of
> >5-10%,) which means that the match isn't all that bad even if the length
> >a bit wrong. For that matter, a dipole has approximately a 70 ohm feed
> >impedance which in itself gives you a non 1:1 VSWR (although, if you bend
> >a bit, as in an inverted V, it will come closer to 50 ohms).
> >
> >Put some numbers to it.  Say you wanted to operate at 14.2MHz, and your
> >antenna wound up being resonant at 13.9 MHz. That's only 300 kHz away,
> >the VSWR is pretty close to 2:1, which your tuner will have no problem
> >matching.
> >
> >You don't want to be too cavalier with the lengths, because the mismatch
> >going to have some power circulating between the antenna and your tuner
> >through the feed line.  The tuner's reactive components are probably the
> >dominant loss in the system, and you want to keep them to a minimum.
> >However, even if you wind up 5% away from the resonant frequency, it's
> >only about 3:1 (into 50 ohms).
> >
> >Spend your time trimming the low band elements (the band is a wider
> >of the center frequency, (3.5-4MHz is 13% bandwidth..)  However, even
> >you're still probably within 4:1 if the wire is even remotely close to
> >right length.
> >
> >
> >Here's the upshot... Get out a tape measure. Cut the wires to about the
> >right length.  Pull it up in the air, use the tuner, be happy.  If the
> >antenna is close to the ground, contemplate making the wires a bit
> >(you can just fold them back on themselves, or wad it up in a ball, to
> >a capacity hat). When I had my multiband dipole 5 feet off the ground, I
> >found that I needed to shorten the wires almost a foot to get the
> >in band.
> >
> >Spend your time on getting the wires high in the air and away from other
> >things.  That's probably more important from a loss/efficiency standpoint
> >than anything else.
> >
> >
> >If you need something to do while it's pouring rain, download a copy of
> >4nec2 (free) or multinec (nominal cost) and plug in a design and let the
> >optimizer find out the exact right length wires.
> >
> >----- Original Message -----
> >From: "Tim Holmes" <>
> >To: <>
> >Sent: Thursday, October 21, 2004 12:06 PM
> >Subject: [TowerTalk] New Member - Need help with an antenna design
> >
> >
> >
> >
> >>Hi Folks
> >>
> >>I'm trying to put some work into my antenna system before winter really
> >>gets going, and I am looking to redsign my multi-band dipole.  I am
> >>strongly considering a fan type dipole cut for the center of the General
> >>band on 80, 40, 20, 15, 10.  I use and IC706MKIIG, and have LDG AT11mp
> >>(I think) auto tuner.
> >>
> >>The antenna will be hung roughtly north south at about 30 feet, and will
> >>have a 20 degree dogleg to the west in the north half.
> >>
> >>I realize that a dipole is nowhere as good as a beam etc, but I cant put
> >>up a beam, and I can put up a dipole, so I need some suggestions, or
> >>links to plans etc.
> >>
> >>Thanks for your time
> >>
> >>Tim Holmes
> >>Amateur Extra Operator
> >>W8TAH
> >>_______________________________________________
> >>
> >>See:  for "Self Supporting Towers", "Wireless
> >>
> >>
> >Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with
> >questions and ask for Sherman, W2FLA.
> >
> >
> >>_______________________________________________
> >>TowerTalk mailing list
> >>
> >>
> >>
> >>
> >
> >
> >
> >
> >


See:  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>