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

[Tim Holmes]

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

TIM


Jim Lux wrote:

>Since you have a tuner, you're in great shape.  The usual problem with multi
>band dipoles is trying to get a decent match on all bands, because the wires
>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 fix
>it.
>
>A couple ideas.. I have a Alpha-Delta DX-CC which is a multiband dipole with
>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, especially
>if you got the pink UV stabilized kind).  Originally (before tuner days), I
>tuned it up using a MFJ antenna analyzer, and it was a bit tedious.  Start
>with the 10m element, adjust, then adjust the 20 element length, then adjust
>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 is
>a bit wrong. For that matter, a dipole has approximately a 70 ohm feed point
>impedance which in itself gives you a non 1:1 VSWR (although, if you bend it
>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, and
>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 is
>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 still
>only about 3:1 (into 50 ohms).
>
>Spend your time trimming the low band elements (the band is a wider fraction
>of the center frequency, (3.5-4MHz is 13% bandwidth..)  However, even here,
>you're still probably within 4:1 if the wire is even remotely close to the
>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 shorter
>(you can just fold them back on themselves, or wad it up in a ball, to make
>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 resonance
>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" <W8TAH at Zoominternet.net>
>To: <towertalk at contesting.com>
>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: http://www.mscomputer.com  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
>>TowerTalk at contesting.com
>>http://lists.contesting.com/mailman/listinfo/towertalk
>>    
>>
>
>
>
>  
>