[Fourlanders] Preamplifiers and Feedline Questions from a Newbie

[Rogers, Ron]

Mike,

I have had some experience with VHF-UHF tower mounted preamps, but
mostly for temporary contest setups. And, I have also used mast mounted,
more permanent preamps for satellite antenna setups.

My experience covers 2 main situations: 

1) Homebrew weatherproof preamp housings  for 222, 432, & 903 Mhz,
switched in and out remotely by transmit sequencing, contain their own
DC power supplies in the housings, and installed inline with systems
delivering antenna power between 200 watts and 1,000 watts. These have
been used during temporary contest installations that saw infrequent
occurrences of local lightning. The preamps used were ARR and DEMI.

2) Commercial tower mounted preamps, that are powered via DC through the
coax, contain their own RF switching circuitry, and rated at 160 watts
RF. For this type of preamp I have used the ARR MSP series with good
results for satellite communications.
http://www.advancedreceiver.com/page24.html 


The ability of any tower mounted preamp to withstand near field EMP from
lighting, I think is a roll of the dice. But, all of the commercial
tower mounted preamps are designed such that when DC power is applied
through the coax, the relays "pull in" and connect the preamp inline to
the antenna. When RF is applied, the switching PIN diodes and transistor
switch remove DC from the relays, thus they "drop out" and bypass the
preamp. So, if you shut off the DC to the coax (or have a DC failure),
the relays then drop out and the preamp is bypassed, thus protecting the
preamp from being fried by any RF going up the coax. 

In this mode the ability of a preamp to withstand a near field strong
EMP would be questionable, but at least is better protection verses
leaving the preamp in the line when un-powered, which would be the
product of an inferior preamp application design. 

This design approach is the same for the home brew tower mounted units I
created, with the exception that I included small DC power supplies in
the housings to supply power to the preamp, and simply interrupted power
to the preamp when the Dow Key relays dropped out during transmit.  
  
 
Ron 
WW8RR


-----Original Message-----
From: fourlanders-bounces at contesting.com
[mailto:fourlanders-bounces at contesting.com] On Behalf Of Dr M J
DiGirolamo
Sent: Thursday, March 22, 2007 9:39 PM
To: fourlanders at contesting.com
Subject: [Fourlanders] Preamplifiers and Feedline Questions from a
Newbie

Dear Colleagues,

  I've been "reading the mail" on this group for some time now.  I am in
the midst of erecting my first tower after being a ham for 39 years.  My
heart favors VHF, UHF and microwaves though I haven't forgotten HF.

   Since my current tower will have antennas from HF to 23cm, at heights
to 90'
feet, I have need for preamplifiers and I have several unsettled issues
regarding feedline and preamps.  At the coaxing of Brian, NX9O, I am
making my first post to this reflector.  I'd like to make contact with
some person(s) who have are experienced with mast mounted preamps.  I am
undecided about which ones to purchase (SSB vs DEM) and how to protect
them from EMP resulting from near lightning strikes.  Not sure this is
even possible?

   Anyway, I'm retired and would appreciate making contact with those
who have time to offer advice.  THEN, one day, I hope to work you on the
air.

73,

Mike DiGirolamo, W4XN
Charlottesville, VA

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