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Fig. 2. The nomograph for the H-p/ane ofthe antenna.
series. The lonnula calculates the
gain using two identical antennas
that are a known distance apart.
The lormula is given in Fig. 1.
Basica lly, all you need are two of
the same yagi antennas and the
ability to measure the distance
between the two, as well as the
power delivered to one antenna
and the received signal lrom the
other.
We set up the two 55-element
yagis so that the front-most directors
on each yagi laced each other
at a distance of 751eet (100 wavelengths
at 1296 MHz). Using a
Kenwood TR-9000 and SSB Electronics
l T23S Transverter as a
signal source. a Bird 43 wattmeter
was put in the line right at the RG­
213 pigtail, meaning it was about
1.5 feet from the dipole-drtven element.
At the receiving antenna,
we mounted a Boonton 92 r1 millivoltmeter
with 5O-Ohm probe and
lots of Bunji cords. The power output
was set at 5 Watts on the Bird
43 and allowed to stabilize for a
few minutes. The r1 millivoltmeter
was also warmed up and set to the
+ 20 d B m range. Steve then
swung the receive antenna back
and forth slightly to peak the indicated
output. AI this point, the
measurement taken from the millivoltmeter
was + 13.5 dBm (22.4
mW).
Plugging this into the formula
resulted in a figure of 265.98 lor
both antennas. One antenna
would exhibit 3 dB less, or half the
gain: 132.99. Ex pressi ng this
number as gain in dB would be 10
IOg10 132.99, or 21.24 db. The
manufacturer claims 21.25 db.
Close enough for you? I would assume
lrom this that Tonna's gain
figures probably aren't overrated.
The pattern is very sharp. FIQ. 2
gives the nomograph lor the Hplan
of the antenna. The 3 dB
beamwidth is clai med to be about
11 degrees. The nomograph and
our experiments wou ld bear this
out. The 55 element F9FT is a
84 73 for RadiO Amateurs • April, 1986
sharp antenna. and while it exhibits
a considerable amount of
forward gain, the Sharpness 01 the
pattern might make it unwiekty lor
the average 23 cm operator. Still,
il you understand the antenna and
its advantages an d disadvantages,
it can make the difference
for your 1296 MHz station. For example,
the K3YTL cootest group
used eight of these during the recent
September VHF aoo Party
and worked 65 stations on 23 cm.
It definitely works!
The Tonna 55-element antenna
costs $70. US Importer: VHF
Sh op. 16 S. Mountain etva.,
Mountaintop PA 18707.
Peter H, Putman KT2B
Morris Plains NJ
MV 220 MAST MOUNTED
GaAsFET 22D-MHz PREAMP
SSB Electronics of tserronn.
Germany. has become the nrst
European manufacturer to come
out with a commercially available
mast-mounted preamplifier for
220 MHz. The 220 MHz band isn 't
available to Europeans and this
represents an unusual bit of marketing
on SSB's part. How much
of a demand is th ere lor 220
preamps? Only time will tell, but
this is a big step i n the right
direction.
Based on the popular MV series
preamps, the MV 220 is basically
a rewOr1(ing of the MV 144
mast-mount preamp. Employing a
single GaAsFET, the manufacturer
claims in excess of 16 dB of
gain. This is considered a 1 kW
through-line switchable preamp,
and although the unit has its own
rf vox type sensed keying, SSB
strongly urges use 01 its companion
sequencer when running high
power levels.
I've always admired the rl and
watertight cases in which SSB
makes their preamps . A small
gasket runs around the lip 01 the
cover to keep moisture out. You 'll
need to make three connections:
antenna, transceiver and de power.
As with all SSB preamplifiers,
the coaxial relays switch in whe n
power is applied. This protects the
GaAsFET in case of power loss, or
in the event 01 slrong lightning
storms. You just cut the power
and it's out ol tha line.
Threaded studs are attached to
the case, so you can bolt the unit
directly up to a 2-inch mast. A
small piece 01tubing and gasket
serve to waterproof the de power
connection cable, which you must
supply. Installation is simple and
takes only minutes. I used this
preamp with my existing ARR sequencer
IOf keying , and had no
problems with false keying, although
water got into one of the
pigtails and froze, creating a high
swr condition. It made no difference
to the preamp as the GaAs­
FET percolated happily along.
On-air tests were impressive.
220 MHz is a sparsely populated
band. and quite often the only signal
you hear on it is weak and
some distance away. In this case,
the preamp makes a big difference,
especially on signals with
lots of aSB. In many cases. my
basic 220 transverter-a "Canadian"
Microwave Modules 22Q.
28---coo1dn't even detect the signals
that came up to 53 with the
preamp switched in, largely due to
the noise figure of the 220 module-around
2-2.5 dB. When
working stations with rapid fading
or flutter, the difference between
hearing the conversation and
hearing big gaps filled with noise
was this preamplifier.
Okay, so it worked. But how
well? Bac k to the lab: Using
Hewlett-Packard test equipment, 1
measured the gain, minimum discernable
signal (MDS) and 1 dB
compression poi nt. On-air tests
indicated that this preamp had
lots of gain. and the measured figure
of 23.2 dB al22O.oo MHz certainly
confirmed it. No doubt, this
is one of the holiest preamps
around-if you go by gain alone.
What abo ut MDS? In my tests. a
signal of - 140 dBm was d etectable
using a 1 Khz bandwidth.
Pretty sensitive. A signal level of
- 130 dBm gave us a 10 dB S +NI
N ratio, so it certainly hears the
weak signals, not to mention
putting my MMT 220 to shame.
Well, what can you expect from a
MOSFET at 22O? A noise figure of
2.0 dB is not shabby.
I was surprised during the 1 dB
compression test. This preamp
does have lots of gain, and on-air
tests indicated thai the strong signal
from Channel 13 was pumping
the preamp adversely. By "pumping,"
I mean a general hash and
splatter of intermod products
head up and down the 220 band
when my yagi was pointed towards
New York City. WithOut the
preamp, these products were unnoticeable.
Incidentally. another
preamp I use here (ARR 220 VDA)
exh ibits the same problem, but to
a lesser degree. It uses a MOS·
FET and also has less gain. I
didn't get to verify its 1 dB point
but the SSB MV 220 came up
shOrt at -3.5 dBm out. What this
means is that the preamp is likely
to have trouble with strong local
signals , on adjacent c hannel s
(and even on an adjacent TV
channel) to the detriment of the
weak signal you are trying to hear.
This isn't the first time I've
observed a low 1 dB compression
point on an SSB product. .Another
MV 144 preamp exh ibited
much the same number, but had
lots of gain. Perhaps the compromise
between gain and compression
point still needs to be
worked out by the folks in Iserlohn.
Incidentally , the gain centers
o n 225.00 MHz, so this
preamp could be employed for
weak signal FM work as well on
223.50 with good results. In fact,
the preamp still had 10dB 01 gain
at210 MHz and 233 MHz-which
is pretty broadbandedt
For those with a preponderance
of weak signalS on 220 to choose
from, this preamp might make the
difference. If you live close to TV
channels 12 or 13. you may find
the "pumping" problem to be a bit
01a headache, but there is a provisian
to adjust the gain internally
by means of a potentiometer. This
pot is located at the input to the
preamp and is part of a resistive
ertenuetor network. The total attenuation
is on the order of 5 dB. I
haven't had a chance to use this
control, but would be tempted to
replace it with 10 dB ot fixed attenuation
and run the preamp at a
net gain of 13 dB or so, which
might help with the overload problem
(and also raise the compression
point above zero). I'll try to
run these tests and report on them
in a luture " Above and Beyond"
colu mn.
SSB Electronics MV220 GaAs­
FET Preamplifier, Price Class:
$180; US Importer: The VHF
Sh op, 16 S . Mountain Blvd.,
Mountaintop PA 18707.
Peter H. Putman KT2B
Morris Plains NJ