Re: Repeater - Radio - Receiver Pre-Selector Recommendations
I received the following question by direct Email and thought
it might be useful to reply back via the group. There I go
thinking again...
> Do you have any recommendations for a 3 cell pre-selector
> for 150-160 MHz with an insertion loss of around 3 dB ?
Yes and No... the obvious answer.
Each and every application seems to be different so I end up
engineering front end filtering pre-selectors based on a
checklist of requirements mentioned below.
1. Fixed single frequency of operation or x-amount of
bandwidth required.
It seems to be an advantage for most people to use receiver
cavity type pre-selection to protect a single frequency
receiver. Repeater input frequencies for a time tended to
be placed in one portion of a band with corresponding high
power transmitters place at/in another portion of the band.
But nowadays... all bets are off. Are you feeding one or
more receivers?
2. Adjacent transmitters and their respective frequencies.
Continuing on from above... I now see a lot of very poorly
planned frequency coordinations. In some cases including my
own... the FCC has licensed relatively high powered transmitters
a very short physical and spectral distance from a number of
receiver input frequencies.
I have one location where the FCC generously granted a large
number of repeater transmitter frequencies less than 200KHz
from the receiver input frequencies. Go figure...
Another mountain-top location where a valley floor located
co-channel user 80 miles distant blows 160 watts from a 200
foot tower station master antenna system and quite often takes
out local portables operating on our system.
Both the above examples are clearly serious head scratchers,
which obviously change the pre-selector requirements.
3. The type of antenna or antenna combiner/distribution
system.
Sometimes you need wide-band antennas and sometimes you want
narrow band antenna performance. In the case of two different
folded dipole layouts... one specs operation from 138-174MHz
while the other brand/type a more conservative 10 MHz segment
of 138-174MHz. No sense making it easy for strong out of band
undesired signals to reach your receiver. Omni or directional
antenna patterns desired..?
4. Options
Do you buy a pre-made pre-selector filter assembly or design
one?
****
So... in reality there is no off the shelf filter that takes
care of all situations. If you don't have to deal with any
of the commercial world land minds... I do like and generically
use a lot of the DCI filters for out of band protection. I've
even had DCI custom make a number of UHF repeater pre-selectors.
I do see a number of wide-band VHF receiver pre-selectors
(Cellwave, Decibel, Sinclair & Pyramid) but I normally don't
use those as repeater site pre-selectors where I need a lot
of performance for portable (HT) operations. Notice I wrote
"normally"...
The Pyramid BPF-1604 seems to be the type of pre-selector
referenced in the original question.
http://www.pyramidcomm.com/pdf/BPNF/4%20Page%20Filter%20Brochure.pdf
I am a Pyramid Dealer and use a fair number of their various
filter products successfully installing in-band and cross band
SVR-200 mobile vehicle repeaters. It's not super easy to get
a duplex frequency agile receiver to happily live spectrally
and physically next to a 60 watt transmitter.
Otherwise it's a custom design and assembly based on the
specific application. As always... the no free lunch rule
applies. The group here is a great place to ask for ideas...
so do ask and we'll all chime in with free or very low cost
opinions... I can be easily bribed...
cheers
skipp
skipp025 at yahoo.com
www.radiowrench.com
