The ARRL antenna book shows a 2:1 SWR bandwidth of less than 300 KHz for an
80m cage dipole with a 6 inch spreader diameter. DX Blaster claims 400 KHz
bandwidth for their 80m cage dipole. I am not sure how big their cage is,
but it would have to be pretty large, or they have some other methode of
obtaining this much bandwidth, assuming that they are measuring SWR at the
antenna input.
To: Bill Aycock <baycock@centurytel.net>
Subject: Re: [TowerTalk] Cage dipole alternatives
From: "Roger (K8RI)" <K8RI-on-TowerTalk@tm.net>
Date: Mon, 13 Apr 2009 01:42:57 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Bill Aycock wrote:
> Rick
> Interesting alternative. I have a question about the concept. As I
> understand it, the "cage" concept needs for the wires to be close enough
> to
>
The "cage" is pretty much a regular dipole with a large length to
diameter (l/d) ratio as given in the hand book.
It's also a very old design and can be found in early literature.
It "to me" is an enlargement to the "fan dipole" concept. IOW it matters
not (electrically) whether the ends are tied together or not. Physically
it makes a big difference as once up the fan dipole is fairly rugged
(depending on construction and only needs one support for each end. It
also represents a *relatively* small profile compared to an actual
conductor of that size. OTOH as I mentioned in an earlier post, it does
not handle ice well.
The cage will work with only two wires, but it is simulating a large
diameter conductor so a few more would be prudent. "I doubt" much would
be gained to go beyond 6 or 8, but I've never seen anything definitive
as to where rapidly diminishing returns becomes a factor. 4 is likely to
provide a reasonable bandwidth.
There, yah went and done it...I had to dig out the books <:-))
To Quote the ARRL Antenna Handbook (Pg 9-4, 20th edition) :"The gain and
radiation patterns are essentially the same as a thin wire
dipole"..."The bow-tie and fan dipole make use of the same Q-lowering
principle as the cage for increased match bandwidth." They use a 6"
diameter cage for an example. The matched bandwidth should be 1.79 times
the matched bandwidth of a single wire dipole for their example antenna.
> act as one *large* conductor. How close fits this concept? In the case of
> a
> Fan with diverging wires, where does the "single large conductor"
> equivalence fail?
>
No where specific and yet it'd dependent some what on the separation
(think bow tie) and the lengths also play a part. IOW cut one for the
higher end of the band and the second for the lower. Add a third for in
between. I've never been able to make the multi-band dipole using wires
for each band to work as they show it. I've always had to use much more
end separation. Adding more wire changes the end effect and capacitance
which results in antennas being some what shorter than single wire
antennas be they multi-band, cage, or fan. With the cage the length
should come out pretty close using the standard formula with the l/d
ratio taken into account.
As to the antenna that originally raised this question with claims of
measured gain of 5 dbd, as I've mentioned before, there could be many
reasons for that figure to have actually been measured when comparing
two dipoles. In "that test" the antenna likely did show that gain
*compared* to the reference dipole, but in the real world the cage is
not an antenna with any essential gain over a thin wire dipole as far as
any antenna book (including college texts) will show.
They are a good antenna, essentially rugged in most aspects and almost
twice the bandwidth of a thin wire dipole (depending on size). They are
easy to build IF you have the room to lay out the wires. Typically they
are not an antenna you want to assemble in a small room.<:-))
73
Roger (K8RI)
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