And how do you engineers tune your towers for the lowest
possible SWR? At the power levels you're running any
reflected power would be measured in the hundreds of Watts -
enough to cause quite a bit of damage to your PA.
Do you notice any changes in SWR with the weather? Since your
ground plane system is so extensive I would imagine after a
heavy rain the impedance, and thus the SWR, of your antenna
system would change.
I believe it's a half-wave, about 760 feet in length.
> In fact, what's more popular for AM
> stations: 1/4 wave or 5/8 wave?
Generally, half-wave is considered ideal, but they come in many different
fractions. Many of the stations at the bottom end of the dial, for
example, could not afford to put up half-wave towers, especially in
directional arrays. Quarter waves are more common there. As for
popularity, though, I think most engineers would prefer 1/2 wave towers,
altough maybe Lou Schneider or somebody can come in and correct me on
A 5/8 wave tower pushes more of the energy along the ground and less to
the sky, but at low angles still produces a good skywave. WSM (650) in
Nashville has a tower that's over 800 feet tall that probably fits this
> With either you have to have
> an excellent ground plane (visions of a couple San Francisco
> AM transmitter sites sitting in the back waters of the Bay
> come to mind; salt water is an unbeatable counterpoise!).
Salt water has the highest ground conductivity available; and the SF Bay
area is blessed with a lot of it. Some of the Bay Area also has high
ground conductivity away from the water, too. This is one reason why AM
stations here do so well.
By the way, none of the big Bay Area stations uses a 1/2 wave tower. The
closest is KNBR, which uses a "capacitive hat" on its 568-foot tower to
achieve an electrical half-wave length. The height is truncated, I
believe, to respect airport approach paths. KGO's towers at the east end
of the Dumbarton bridge are only 220 feet. KCBS's up north of Novato are
around 500 feet. KFRC does extremely well with a 400+ foot 1/4 wave tower
in Berkeley. By the way, KCBS blasts the equivalent of about 200kw in the
direction of Los Angeles, down the length of the bay. The signal at night
in Southern Cal is like a local.
You'll need a better engineer to answer your VSWR questions.
: I believe it's a half-wave, about 760 feet in length.
: > In fact, what's more popular for AM
: > stations: 1/4 wave or 5/8 wave?
: Generally, half-wave is considered ideal, but they come in many different
: fractions. Many of the stations at the bottom end of the dial, for
: example, could not afford to put up half-wave towers, especially in
: directional arrays. Quarter waves are more common there. As for
: popularity, though, I think most engineers would prefer 1/2 wave towers,
: altough maybe Lou Schneider or somebody can come in and correct me on
In reality 1/4 wave towers are the norm. Of the 100's of AM stations
that I measured for NRSC Compliance last year on Texas, New Mexico,
Oklohoma, and Lousiana there were only 3 that used any thing other than a
1/4 wave tower. Those two were 1/2 to get the height needed for the side
mounted FM antenna. One was a Franklin design that allows you to contol
the takeoff angle of the signal for Sky Wave control.
Some directionals that I have worked on latley are in deed marginally
engineered. 1 5/8" line for 50Kw Transmitters. Medium power towers with
7/8" line. This is asking for truble and trouble did indeed strike.
Taking out the Referance tower, causing the remaining power some 35 Kw to
flow into a 7/8" line resulting in burned line. At that power level the
line go like fuse. By the time the Mangnphase triped the transmitter off
the damage was done. The transmitter of course was fine but the
undersized 5 tower directional was gone. After being in operation only 3
I replace the 1 5/8" line with 3" and the two medium power towers were
upgraded to 1 5/8". That was 1987 and it's been on since then with no
US 90W Communications
CE WPGC AM/FM
K7QA or WPGC955 @ aol.com
Sometimes you can lose a tower or have another major event that only
causes a small change in common point impedance. KCBS lost one of our 4
towers back in 1988 during a violent windstorm - this was the highest
powered tower in the array - 22 out of 50 Kw. The feedline shorted
against the ATU backplane as the tower went down, and the feedpoint
shifted to about 60 ohms with a little reactance. We were running a MW-50
with fully functional VSWR protection, and it was perfectly happy feeding
this until we were able to get out to the site the next morning.
Tom McGinley K7QA @aol.com CE WPGC AM/FM