Several NAB engineering papers have be written with field tests on the
caged feed and demonstrate that whether you feed the cage or if merely
connect the cage to the bottom of an insulated tower both the field
strength and bandwidth remains the same. All agree that for a lower Q
cage fed antenna the station actually sounds better than higher Q feed
systems which could and often reduce the band pass fringes so the signal
"sounds narrow rather than fat" on the BC frequency assignment. I know
the later is anecdotal but many broadcaster who use the cage prefer them
over the straight feed.
On 160 if you wish to operate across the band and don't have a remote
tuner to accomplish this a cage can help.
Herb Schoenbohm, KV4FZ
On 1/26/2012 10:25 AM, W9UCW@aol.com wrote:
> During the 1980's we had a 120 foot, insulated base tower with an
> appropriate radial system. I wanted to improve it's performance and increase
> its
> bandwidth. I also wanted to add an 11 foot conical monopole to our collection
> of antennas.
> With the help of friends, we mounted the 14 to 60 MHz conical monopole on
> top of the tower. Then we "caged" the tower from top to bottom at 10 feet
> in diameter using #12 wire and 10' PVC pipes as spreaders at three levels.
> On 160 it was fed as a folded monopole with the tower grounded and it was
> below 1.5 to 1 SWR for most all of the band. On 80 where it was voltage fed
> as an insulated base, very fat half wave monopole, it was below 1.5 to 1 SWR
> for nearly the entire band. The cage doubled the bandwidths.
> IMHO, having also worked with using a "cage" as the gamma rod on grounded
> towers in several cases, I would say the positive effects are worth the
> trouble, Dan. It's hard to predict the results on your inverted "L" but I
> would urge you to try it.
> BTW, I had a remotely controlled tuner/matcher at the base of that big fat
> vertical. It allowed us to switch configurations, tweak the capacitance
> and inductance settings and either connect or disconnect the coax up to the
> conical monopole on top. I wish we still had that set up.
> 73, Barry, W9UCW
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|