[Elecraft] Big Coax on Dipoles

[Wes Stewart]

Re: 150:1 SWR.

I have an inverted vee style dipole consisting of two wires fed in parallel at the feed-point, one cut for 80/75 (depending on some jumpers at the ends) and the other for 40-meters. The feed is at about 45 feet above ground and the low-freq dipole ends are at about 25' above ground.  The wires are displaced horizontally (fanned) about 25 degrees from each other.

I have a pretty accurate model of this antenna, that has be confirmed against actual measurements on the two bands using my N2PK vector network analyzer that was calibrated at the end of a 50' run of cable necessary to get from the antenna to ground.

The tower is about 125' from the shack so the total run of RG-213 is about 200'.

The modeled worst case SWR occurs on 20-meters where it is about 32:1.  Including the transmission line loss decreases the SWR to under 5:1 over all of the ham bands from 80 thru 6 meters, with the worst case still being 20-meters.  The worst case line loss is a bit over 8 dB, with 20 and 10-meters being the culprits. 

Since I have a HB three-element Yagi for 20, that band is not an issue and I can work all of the other HF bands using only my L4-B "active tuner".  I've also worked about 120 Q's on 6-meters using the K3 (no tuner) and have about 20 states worked and 5 DXCC countries, including a couple of JAs on that band using the same antenna.

(Editorial mode on: The JAs were worked on Field Day and I worked them about an hour apart.  The first one was still calling CQ without success when I worked the second.  So the band was open for DX but no one (except me) was listening.)

However, this is in no way an endorsement of using such a system as an all-band antenna, even one fed with ladder line.  These antennas are at best compromises, with generally unknown performance characteristcs, and at worst not much better than dumy loads.

Wes Stewart  N7WS


--- On Wed, 7/22/09, Steve Ellington <n4lq at carolina.rr.com> wrote:

From: Steve Ellington <n4lq at carolina.rr.com>
Subject: Re: [Elecraft] Big Coax on Dipoles
To: "Jim Brown" <jim at audiosystemsgroup.com>, "Elecraft List" <elecraft at mailman.qth.net>
Date: Wednesday, July 22, 2009, 7:57 AM

The feed point on some of those bands would see a 150:1 swr (no kidding)! 
Your efficiency would be about 5%. Your tuner would still  see a low swr and 
it will be an easy match because of the tremendous loss in that 100 pounds 
of coax. Sure you can work stations, with that height you could QSO with a 
light bulb,  but you would be 100 times stronger using ladder line plus you 
wouldn't need 95 pounds of sand to hold up that lossy dummy load coax.
I'm feeding a 200' doublet with ladder line then to a 4:1 current balun next 
to the K3. The K3's internal tuner finds a match on 160-6 meters and I have 
very little power loss on any band. Now if I could just find a fertilizer to 
make my tree grow another 50'!

Steve
N4LQ
N4LQ at carolina.rr.com
----- Original Message ----- 
From: "Jim Brown" <jim at audiosystemsgroup.com>
To: "Elecraft List" <elecraft at mailman.qth.net>
Sent: Wednesday, July 22, 2009 1:03 AM
Subject: [Elecraft] Big Coax on Dipoles


> On Fri, 17 Jul 2009 09:19:03 +0100, Geoffrey Mackenzie-Kennedy wrote:
>
>>orgetting about RF performance for a moment, wouldn't the weight of RG-8/
>>RG-213 create a problem unless the antenna is supported at its feedpoint ?
>
> I have four fan dipoles suspended at 100-120 ft between tall trees
> (redwoods), all fed with RG8 or RG11. There is no problem supporting the 
> big
> coax. The keys to success are robust mechanical construction so that you 
> can
> establish high lateral tension. If one of the objects you're suspending 
> from
> is a tree, one end needs a weight and pulley to adjust to wind conditions. 
> I
> use 6.5 gallon water jugs filled with dry sand (roughly 95 pounds), and 
> the
> trees are typically 140-220 ft apart. I use 5/16-inch dual woven poly 
> rope,
> #10 solid copper, and robust insulators. Do they sag? Sure -- at least 10 
> ft
> on the long dipoles. Does it matter? No.
>
> One of the side benefits of the big coax is that my 80/40 fans (at right
> angles) work quite well on 30M, 17M, 12M, and 6M, thanks to the lower loss
> with moderate degrees of mismatch. With relatively litte time on 6M, I've
> made nearly two dozen QSOs of more than 2,500 miles with my K3 and one of
> those dipoles.
>
> 73,
>
> Jim Brown K9YC


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