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Better FM Antenna Performance
We Conclude Our Series of Questions and Answers
With Leading FM Consultants
by Richard Fry
How can you get the most out of your FM antenna system?
To provide some insight into this subject, three prominent broadcast
consulting firms were asked to respond to 15 topics about FM antenna
performance. With this issue, we conclude their replies. Past articles
are available at www.rwonline.com under the tab "Better
FM Coverage."
The participants are Ben Dawson of Hatfield &
Dawson; Bob Culver of Lohnes & Culver; and Don Markley of D.L.
Markley & Associates. Note that any consultant would need specific
site and application information to provide an accurate recommendation
for a given situation.
Topic: Comment on building penetration issues that
confront you as a consultant.
Dawson: We make a lot of building penetration
measurements at land mobile frequencies where talk-in can be particularly
critical because of the limited power budget. For broadcast FM we
have some pretty valid assumptions we normally use, based originally
on measurements.
Culver: A dB is still a dB and building penetration
is just the ratio of the signal impinging on the building to that
inside, just a dB ratio. Add 3 dB outside and everything inside
goes up by 3 dB too.
Now if you are concerned about multipath, if the
antenna does not change and the propagation path(s) do not change,
then a dB is still a dB. If you happen to be on the threshold of
service, on the edge of the FM signal-to-noise curve, 3 dB outside
could sound like a lot more inside, you are in the area of non-linear
audio performance vs. RF signal.
Look at any receiver performance curves. It may be
that with more (or less) power, the audibility of an artifact, multipath
included, is masked. But, if you change the power up and
down, the multipath effects will stay the same, you will have the
same direct and reflected signal ratios and phases.
You probably could do a lot better to improve audio
at the threshold of service by turning off the stereo. That is worth
about 23 dB SNR.
Markley: For best building penetration, we
suggest the highest possible ERP as opposed to adding height greater
than that assigned to the class of station and then decreasing power
as needed to comply with the rules and the maximum distance to the
protected contour.
Topic: What is the best choice of antenna polarization
for various types of terrain and urban environments (H only, V only,
elliptical, circular)?
Dawson: There may be circumstances where plane
polarized antennas are more desirable, but if there are, we haven’t
found any. CP and mixed polarization are very satisfactory.
Culver: As I recall, H-pol has a bit of an
advantage because of less reflections. But today we have a lot of
vertical (or at least slanted) antennas on cars, so the vertical
component is a help.
You want the real answer to improved performance?
Diversity receive antennas. But then I’m not answering this for
the transmission antenna folks.
When we discussed this with the folks from one of
the big two auto manufacturers, all these topics were discussed.
The auto folks said, "Yes, but that costs money, and beside
the antennas look bad and make noise. Isn’t there any way to have
good audio performance without the antennas?"
My answer was, "Yes, throw away the antennas
and the radio and just put in cassette players."
That was in the days before automobile CDs.
Markley: We fully believe that circular polarization
is the best for all FM service with a single station. In some cases
where boosters are to be used, we have had significant success using
horizontal polarization for the main station with vertical polarization
for the booster signal.
Topic: Comment on the importance of axial ratio
in a circularly polarized antenna. This would address parity
of the radiated waves in all polarization planes, not just H&V.
Dawson: If receiving antennas were CP, this
would matter, but since they are generally randomly linearly polarized,
it appears to be relatively unimportant.
Culver: Interesting question. Any side-mounted
antenna that has different H-pol and V-pol azimuth patterns will
have a big axial ratio all around the pattern, except where the
two fields were the same. Just like the broken watch, at least it
is right twice a day (and the axial ratio is near zero at two or
more points but not many).
The only inherently low axial ratio antennas I know
about are panel antennas, and the best of these is the CBR. The
reason for this is that the radiating element is the round cavity
and it is identical for all polarizations over its azimuth and vertical
patterns (at least to the symmetricity of its cavity shape).
But does the axial ratio mean much? It would if you
had a C-pol receiver antenna and wanted to reject reflected (reversed
polarity) signals.
Markley: We suggest fully circular polarization,
not vertical and horizontal. In other words, we prefer the field
to be the same for all angles of antenna field rotation.
Topic: Comment on placing the antenna on a short
tower close to the city of license vs. on a tall tower further away.
Culver: Trade-offs! Do you really want to put
a 100 kW FM station in a populous area and deal with the RF exposure
zoning concerns that will come crawling out of the woodwork?
Then there is the issue of FM blanketing. You will
have to deal with all of the complaints for the next year from anyone
within a few miles and if there are 10,000 people in that area and
they get on your case, you will be busy.
I would much prefer to build a tall tower (or lease
space) about 10 miles out, where land may be cheaper, people are
fewer and perhaps not so feisty, and still put 90+ dBu in the city,
20 dB above principal community coverage, than to put 20 dB more
in the city (and thereby have 40 dB more signal above superior service
than just 20 dB above that level).
Markley: To evaluate the problem of tall tower
with lower ERP and short tower with full ERP, it must be assumed
that, in each case, the maximum facilities for the class of station
are used. Then this becomes a judgment call.
In open areas with numerous small communities to
be served, greater height is desirable. However, if the desired
market is a single major city and service at a greater distance
is a secondary goal, the greatest ERP is desirable to penetrate
buildings as much as possible.
In any case, continuing to increase height seems
to be something to be done with caution. You shouldn’t just keep
going until the ERP becomes something like 100 watts. That would
be great for communicating with planes, but poor to get into even
a house.
Radio World thanks the consultants who participated
in this series of questions and answers.
Richard Fry is a retired FM applications engineer
with almost 35 years of service with major U.S. broadcast transmitter
manufacturers. Reach him via e-mail to rfry@adams.net
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