Have a question that you would like to see answered? Send it in to the
webmaster, care of ppfma@ppfma.org.
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How much power should I be using to access the repeater?
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Answer: Let's start with the FCC rules. One rule states that the maximum
legal power is 1,500 watts. Another rule states that an operator must use
the minimum power necessary to communicate. So, to translate that to
a practical answer, you should be using enough power to make your signal a
full quieting signal into the repeater (but not more than 1,500 watts).
As far as the repeater is concerned,
the repeater is designed to relay signals from HTs (about 5 watts), mobile
stations (typically 30-50 watts) and base stations (may run significantly
higher power). Under normal conditions, and if you're within about 20
miles of the repeater, 5 watts on an HT should be enough. If conditions
deteriorate due to wind, noise at the repeater site, or other interference,
an HT may not be effective. You may need to increase effective power up to
50 or 100 watts to overcome some noise. (To increase effective power,
you can either increase the power output in watts, or use an antenna with
a higher gain (e.g., 7 dB vs 3 dB).
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What is a duplexer, and how does it work? How does a duplexer differ
from a diplexer?
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Answer: Simply put, a duplexer is a device that allows a single
antenna to be used for a repeater. Since a repeater must simultaneously
transmit and receive (on two separate frequencies), the duplexer prevents
the transmitted signal from feeding back into the receiver. There are
several designs, but basically there are band-pass and band-reject filters
on both the transmitter output and on the receiver input. On the transmitter
output, the band-pass filter is tuned to the transmitter output frequency
(e.g., 146.970 MHz in the case of a PPFMA repeater), and also on the
transmitter output is a band-reject filter tuned to 146.370 MHz, the
receiver's frequency. On the receiver, there would be a band-pass filter
tuned to 146.370 MHz, and a band-reject filter tuned to 146.970 MHz. There
usually are multiple filters, or cavities, in series to improve the pass and
reject qualities.
In reality, it's impossible to completely eliminate the transmitted signal from getting
back into the receiver. The filters in a duplexer greatly reduce the
signal from the transmitter, isolating the transmitter from the receiver
by as much as 80 - 90 dB.
| | Pass | Reject |
| Transmitter filters | 146.97 MHz | 146.370 MHz |
| Receiver filters | 146.37 MHz | 146.970 MHz |
To explain how it works would require extensive writing, but there
is an article on the subject written by David Metz, WAØAUQ, of the
SouthEast Iowa Technical Society, on their web page at:
http://www.seits.org/duplexer/duplexer.htm
The difference between a duplexer and a diplexer is also noted in the
article. Basically, while a duplexer is designed to allow a single antenna
and feedline to be used while simultaneously transmitting on one frequency
and receiving on another relatively close frequency, the diplexer is a device
that lets you share a single antenna and feedline with two transceivers. The
duplexer passes and rejects very narrow bands of frequencies. A diplexer is
designed to pass two separate, but wide, bands of frequencies. On one input
to the diplexer, it may be tuned for 2 - 150 MHz, while another input may be
designed to pass 400 - 900 MHz.
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Sometimes I can hear hams talking on one frequency, while I'm listening
to another frequency, which is close to where the other hams are talking.
What's happening, and what can I do about it?
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Answer: Let's start with the FCC rules. One rule states that the maximum
legal power is 1,500 watts. Another rule states that an operator must use
the minimum power necessary to communicate. So, to translate that to
a practical answer, you should be using enough power to make your signal a
full quieting signal into the repeater (but not more than 1,500 watts).
As far as the repeater is concerned,
the repeater is designed to relay signals from HTs (about 5 watts), mobile
stations (typically 30-50 watts) and base stations (may run significantly
higher power). Under normal conditions, and if you're within about 20
miles of the repeater, 5 watts on an HT should be enough. If conditions
deteriorate due to wind, noise at the repeater site, or other interference,
an HT may not be effective. You may need to increase effective power up to
50 or 100 watts to overcome some noise. (To increase effective power,
you can either increase the power output in watts, or use an antenna with
a higher gain (e.g., 7 dB vs 3 dB).
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