The
W8IO / WB0DGF Antenna Site
Mast Mounted 144 MHz Preamp Project
(updated 18 Oct 2014)
Last year I was able to finally get my tower and 144 MHz long
Yagi
up on the roof and back on the air again. At the end of 2013
I
was also able to buy a Tokyo Hy-Power HL-350Vdx amplifier for 144 MHz.
I started working stations off meteors on FSK441 and even a
few
stations on JT-65b. However, after several attempts to work
Sky,
VE1SKY, I realized that I need a better preamp for reception.
Sky
could hear me, but I couldn't hear him! I decided that I need
to
design and build a mast-mounted preamp for 144 MHz and place on my
tower near the beam. I am documenting my process to help
others
in similar situations.
Preamp
System Design
The preamp system design for this project can be one of 5
different styles. This article from KL6M sums up
the choices very nicely. Also, K6VHF
gives good advice. My mast mounted LNA will likely be similar to KL6M's
#2 or #4 configuration. I'm not sure that I will need an
isolation relay for the input of the preamp. The high power
relays that I have chosen to use on the tower have at least 100 dB of
isolation already. But, I could switch the input and/or
output of
the preamp into a 50 ohm load using a small inexpensive Axiom DPDT 12
VDC relay from Mouser.
High-Power
Relays
Over
the past few months, I have been purchasing high-isolation, high RF
power relays for this project. Some have been with 12VDC
coils
and some have been with 24/28VDC coils. Here is a photo of all of them.
After measuring the insertion loss and isolation of these, I have
decided to keep the ones with 12VDC coils and sell the ones with 28VDC
coils. The 2 relays on the right side of the photo above are 28 VDC.
The remaing relays are (from bottom clockwise) SSB
Electronics
HF-402 DPDT (purchased for $80 at a hamfest in Arizona), Ducommun
D2-116B10 ( purchased for $65 plus $10 shipping on eBay ), and 2 each
Dow Key Microwave 402_220102A (both purchased from the
Philippines on
eBay for $70 and $40 shipping). The Ducommun and Dow Key Microwave
relay are the best, with less than 0.05 dB insertion loss on 144 and
432 MHz and greater than 100 dB isolation on 144 and 432 MHz.
The
HF-402 is about the same with only about 80 dB isolation on 144 MHz and
less at the higher frequencies.
If
you are reading this and are thinking about building a mast mounted LNA
for yourself, here are some suggestions for finding high power, high
isolation relays. Go to eBay and search for one of the following:
A
lot of the search results will include SMA connector relays.
Unless you are looking for relays for 1296 MHz and above,
disregard these and search the results for Type N. Also, some
of
the search results will include latching relays or transfer relays.
I also recommend disregarding these and concentrate on the
"failsafe" type. The failsafe type has NC, Common and NO labels on the
connectors. Transfer relays have a built-in short between the
NC
connectors inside a DPDT system. Transfer relays usually have
4
connectors in a square configuration. The 12 VDC relays are hard to
find. If you do
find 28 VDC relays, these are easily used in a 12 VDC system by using
the W6PQL voltage converter. Watch out for relays with connectors for
the DC control power. These sometimes have proprietary pin
outs
and you can't find the circuit diagram on the manufacturer's web site.
The good relays usually sell between $50 and $150 each.
Sometimes
you can find one for less than $50. I would avoid paying more
than $100, as you can buy a Tohtsu CX-600N for about $95. The insertion
loss of the CX-600N is good but the isolation is only about 35-50 dB.
Preamps
The
preamp for this project should have a noise figure of less than 1 dB
and have gain greater than 15 dB. Ideally for Meteor Scatter
and EME, the NF should be less than
0.5 dB and gain greater than 20 dB. Since I will be
transmitting with at least 300 watts, I need to use a "receive only"
preamp that requires external switching. My choices for a
NON-RF-switched 144 MHz LNA are:
The ARR P144VDG is a GaASFET with 0.5 dB NF and 24 dB gain.
It
uses older technolgy, but does a very good job at a reasonable cost.
The DEM L144LNA comes in either pre-assembled or kit form. It uses the
newer PHEMT technology and delivers less than 0.5 dB NF and better than
17 dB gain. The WA2ODO preamps also use the PHEMT technology,
but
these can only be ordered from Pete WA2ODO. He usually
publishes
a list of available units in an email on the Test Equipment
Trader email list. His preamps usually have less
than 0.25
dB NF and greater than 20 dB gain, however they are not unconditionally
stable and must operate into a 50 ohm load at all times. The SSB
Electronics is the top of the line preamp and uses PHEMT technology for
less than 0.25 dB NF and greater than 20 dB gain. They are
currently available through Vibroplex in the USA.
Sequencers
Since
this design requires external switching, a sequencer is required to
apply power to the relays, preamp and key the power amplifier. Here are
my choices for a sequencer:
More than likely, I will be using the W6PQL Amplifier Control Board
(version 6).
The sequencer controls the timing of almost
all control board functions; the typical timing for an amplifier
control circuit goes like this: upon receiving the transmit command
from the radio (PTT), do the following:
- Event 1 - While holding off RF
output from the radio, switch the antenna relays to the Normally Closed
"through" position and remove power from the preamp
- Event 2 - 50 milliseconds after
event 1, key the amplifier
- Event 3 - 50 milliseconds after
event 2, enable RF output from the radio
Upon switching back to receive,
these events are performed in reverse order, spaced apart briefly in
time (50ms).
In more detail:
When the control board is switched on,
the LNA also powers up. It immediately generates a negative
ALC voltage, which is fed to the radio to prevent it from releasing RF.
When the radio is first keyed, no RF is sent up the transmission line
because of this ALC voltage.
At the same time (key-up), the
first thing the control board does is bypass the LNA (event 1 of it's
internal sequencer). In this example, it does this by shutting down the
FET
switch feeding power to the LNA and tower relays.
At event 2, you can key up an
amplifier.
At event 3, the ALC is released
and RF is allowed to be generated by the transceiver.
When the control board is manually switched
off, so is the LNA, and the radio operates normally (there is
no ALC blocking signal generated). However, the ability to key the
amplifier is lost with the control board switched off. In the case of
the Tokyo HL-350Vdx, I would assume that the preamp part of the
amplifier should ALWAYS be turned "OFF" because if the control
board is switched off, the radio transceiver cannot switch the
amplifier from receive to transmit - thus destroying the HL-350Vdx
preamp when you try to transmit.
Enclosures
The
size of the outdoor enclosure should be large enough for all of the
relays, preamp, coax cables and termination blocks.
References:
Comments
are welcome!
contact
Roger: email to
rgcox2
(at) gmail.com
Roger Cox - Spring Lake, MI
