Introduction
This article
includes information on improving the sensitivity and noise temperature of
receiving systems, thus improving the quality of weak VHF and UHF RF signals.
There are various ways of achieving this depending on the particular
circumstances. Emphasis is placed on reducing the internal noise floor, and
increasing the signal-to-noise ratio.
Tunable RF
preamplifiers
Most tunable RF preamplifiers use 'set and forget' small input and output
trimmer capacitors. For TV FM DXers, this is very inconvenient because unlike
50, 144, and 432 MHz hams, we are interested in several different frequencies.
Hence, varicap or varactor tuning is very useful for rapid tuning.
Tunable RF preamplifiers are useful for reducing or eliminating
cross-modulation and intermodulation distortion (IMD) problems. Also, by only
amplifying a relatively small RF bandwidth, the dynamic range is improved.
Why use a
preamplifier?
DXers generally only need to use a RF preamplifier if their TV or
scanner/tuner/receiver has relatively poor or mediocre RF sensitivity. The
second reason is because of long cable lengths, which can result in heavy signal
attenuation if a preamplifier is not used at the masthead.
If you live in a high noise area, where the external man-made and atmospheric
noise floor is greater than 4db at 50 MHz, preamplifiers will offer little or no
improvement to the signal to noise ratio. For example, man-made electrical
interference from neighbors is often high enough to negate the benefits of any
low-noise RF preamplifier. Also, on certain days, power line noise can be as
high as S5. When external noise is this high, indoor or masthead preamplifiers
will not improve weak signals.
Apart from re-locating to a rural area, little can be done to combat this
problem. Highly directional antenna systems, coupled with appropriate
polarization, can often reduce man-made QRM. Receiver noise blankers and phase
cancellation can also help, but these techniques are beyond the scope of this
article.
At 45-220 MHz frequencies, man-made noise sources will not be lower than 2dB
in city or suburban areas. Masthead TV preamplifiers with noise figures lower
than 2dB are not really beneficial at VHF, because the antenna receives a
constant background noise of 2dB or more. Only in quiet rural areas will
background noise be below 2dB at VHF frequencies.
If narrow IF receiver bandwidths are used, for example 2.4 KHz SSB, the noise
floor is much lower, hence masthead preamplifiers with noise figures below 2dB
are often beneficial at VHF. For example, 144 MHz weak signal hams often use a
masthead preamp, which have a noise figure of typically 0.7dB.
At UHF frequencies, background noise is typically no greater than 2dB, hence
masthead TV preamps with a 0.5dB noise figure can be beneficial.
RF Preamplifiers are not
always needed
Just because a pre-amp can increase the S meter on your receiver, there is
more involved. If after installing a pre-amp, you may find spurious signals
appear all over the dial! In this case the pre-amp overloads the RF and mixer
stages. The probable cause(s) are excessive pre-amp RF gain, bandwidth, or
inferior bipolar design, etc.
A poorly designed pre-amp can cause receiver problems, even though it might
not degrade the basic performance of the receiver by causing overloading. The
pre-amp could have high noise, and this could make weak signal reception worse
than without the amplifier. Also, the pre-amp might be unstable
(self-oscillating), which can cause all manner of unmodulated carriers to
appear.
TV tuner
sensitivity
Early TV sets, circa 1940, had very poor RF sensitivity. Bob Cooper recently
commented that a receiver of that era required around 1,000 microvolts to
produce a grainy image on the small screen, and RCA was recommending 5,000
microvolts. A modern TV set with 50 microvolts will produce a far better image
than the 1936 version with 1,000 microvolts.
In the 1960's, TV tuner noise figures started to gradually improve, but were
still inferior to some of today's sets. Typical TV tuner noise figures were
around 10-13 dB. For this reason, low noise preamplifiers were more essential
for 1950's and 60's TV sets.
The typical TV tuner noise figure for modern sets is 6-7 dB at 45-220 MHz,
and 10 dB at UHF.
For TV DX, the author uses a HS Publications D100 varicap 40-230 MHz TV
tuner/RF converter. The D100 uses a Toshiba EG522F Mosfet varicap TV tuner. In
most cases, I have found that a 2dB Mosfet tunable 45-70 MHz pre-amplifier
offers little or no improvement on weak video signals. Why? Because at 45-70
MHz, the RF noise figure of the EG522F TV tuner is low enough, hence external
noise becomes the limiting factor. This can be proved by the simple test of
plugging an outdoor aerial into the TV tuner's input, while watching a blank TV
channel (noise on the screen). If noise on the TV screen shows an obvious
increase, the tuner noise level is low enough, and hence a preamplifier will
produce little or no improvement on weak signals. If however no increase in
noise is observed, a preamplifier will be essential.
At frequencies above 88 MHz, external man-made and atmospheric noise is
lower. For this reason, low noise pre-amplifiers are generally more beneficial
for band 3 (170-230 MHz), and especially UHF TV.
During the early 1980's, the German company Telefunken introduced the ET021
Mosfet varicap TV tuner. The ET021 featured Mosfets in the RF and mixer stages
of the VHF section, for use in areas subject to adjacent channel selectivity
problems. Typical figures quoted for an interfering signal two channels away
from the desired signal and to give a 1% cross-modulation on the desired signal
are 100 mV as compared to a conventional varicap bipolar tuner with 25 mV (band
3 TV channels). The ET021 was a revolution in terms of strong signal handling
and low noise RF performance.
The Toshiba EG522F varicap VHF/UHF TV tuner was introduced around 1987, and
is also excellent in terms of strong-signal handling and low noise performance.
Other Mosfet and GaAsfet TV tuners are currently available, for example, around
1989, Toshiba introduced a 3SK97 GaAs-MES-FET TV tuner.
88-108 MHz FM tuner
sensitivity
The author's main FM DX tuner is the Onkyo T-9090 II. Since aligning all the
RF and IF coils, the sensitivity is now optimum. Now that the tuner is quite
sensitive, a 2dB noise figure BF981 pre-amplifier will only offer a small
improvement on weak signals. On the other hand, if a tuner is relatively
insensitive or needs aligning, a 2dB noise figure preamplifier can offer a
significant improvement on weak signals.
During times of high external man-made or atmospheric noise, I find that the
use of a low noise preamplifier, usually offers no improvement on the 88-108 MHz
FM band. For this reason, a sensitive FM tuner, which has optimum alignment,
usually will not need additional external RF pre-amplification.
The T-9090 II features a digital signal strength meter. By tuning to a blank
channel, and connecting an external FM aerial, the signal strength meter
indicates a higher dB reading. Under normal low noise conditions, the increase
is approximately 2dB. If the meter increases by more than 3-4dB, external noise
levels are high, hence a preamplifier will produce no improvement.
When using a preamp for 88-108 MHz FM DX, it is usually best to use the
'local' RF setting on a FM tuner. By using 'local', the tuner's RF sensitivity
is slightly attenuated, typically by 10dB. This means that the total RF gain is
reduced, hence dynamic range is improved.
By using the 'local' RF setting, the RF preamplifier will then provide the
predominant noise contribution and signals will not be masked by mixer noise.
88-108 MHz FM scatter signals will average 1 to 5 microvolts typically at a
distance of 250 miles, requiring a 0.5 microvolt sensitivity receiver and a
10-12 dBi gain antenna.
RDX Labs UA-700 wideband
VHF pre-amplifier
In addition to the BF981 preamps, I also use a RDX Labs UA-700 (designed by
Jim Dietrich, Kansas, USA) wide-band 40-230 MHz VHF GaAsfet pre-amplifier.
Considering the RF input is untuned, the overload and image rejection
specifications are very good. I use the UA-700 indoors, for permanent connection
to my Icom R-8500 scanner. This effectively lowers the R-8500's RF front-end
noise figure from (~ 6-8 dB) down to 2 dB.
The gain of the UA-700 preamp is 10dB at 45-108 MHz, and 15dB at 150-220 MHz.
Low to medium RF gain is ideal for minimising preamp overload. This is partly
why the UA-700 has unusually good overload immunity.
Cross-modulation and
overload
Mosfet TV and FM tuners offer superior performance in terms of strong-signal
handling and freedom from cross-modulation. Reference to the circuit diagram
will reveal what types of transistors are used in the RF stage. For example, the
ONKYO T-9090 II uses Toshiba 3SK114-Y dual-gate Mosfets (1.4 dB noise figure)
for RF amplification. Another advantage of Mosfets is their superior low noise
performance.
High quality scanners, for example, Icom R7000/7100/8500/9000 and AOR AR-5000
VHF/UHF scanning receivers all feature GaAs-MES-FETs in the RF front end, hence
strong signal handling is good.
The gain of any RF pre-amplifier should be usually no more than 20dB. Between
10-15dB gain is usually the best compromise in terms of strong signal handling.
For most domestic DX installations, a low-medium gain/low noise pre-amplifier is
ideal.
Another important key to minimizing overload is to only amplify a small
bandwidth. My tunable BF981 88-108 MHz pre-amp, when peaked has a 2 MHz
bandwidth. This means that only a small portion (2 MHz) is amplified of the
total 88-108 MHz FM band. Overload problems are thus greatly reduced.
Total receive system noise
figure
High gain antennas, low loss cable, and good receivers are the most important
equipment for DX reception. RF pre-amplifiers are generally the least important
consideration for serious 40-108 MHz DX work. However, because external noise
levels are lower above 108 MHz, RF pre-amplifiers can offer considerable
improvement to weak signals. For this reason, most 144 MHz and 432 MHz hams use
low noise Mosfet or GaAsfet RF pre-amps. Generally speaking, mast mounted
pre-amps are better for DX work above 108 MHz.
Because I use relatively short runs of low loss Hills DSC2.1 75 ohm coax
cable (2.3 dB loss @ 100 MHz per 100ft), all my Mosfet pre-amps are used indoors
near the receiver. Low loss coax cable is essential if the pre-amp is used
indoors because every dB of coax cable feedline loss in front of the
preamplifier will add that number of dB to the noise figure of the receive
system. For example, if our antenna balun has 1dB loss, and the total length of
coax cable has 3dB loss, we have 4dB signal loss (3+1=4). If our receiver has a
noise figure of 7dB, we add 4dB, which gives us a total of 11dB. This figure
(11dB) represents our total receiving system noise figure! In contrast, if we
use a 2dB noise figure preamp at the antenna, which is preceded by a 1dB loss
balun, our total system noise figure is only 3dB! Quite an improvement compared
to 11dB!
Remember that a 6dB improvement is equivalent to making the original antenna
system four times as large or going from one to four yagis!
At UHF, if a TV tuner has a noise figure of 10dB, the addition of a 0.5dB
masthead preamplifier will improve the total receiving noise figure by
approximately 9dB! This underscores why masthead mounting of UHF preamps is
important.
By using a pre-amp indoors, only the noise figure of the receiver is
improved. For example, a typical VHF receiver or scanner has a noise figure of
approximately 6-8dB, depending on the RF front-end. By using a 2dB pre-amp, the
effective noise figure of the receiver is lowered to approximately 2dB. With a
mast-mounted pre-amp, the first active transistor in the pre-amplifier sets the
system noise figure, after losses associated with the input balun.
The BF981 pre-amp can be used at the masthead. You will need to run a
shielded wire containing the DC tuning voltage for the varicap diodes, up to the
masthead. Make sure you have good RF de-coupling at the receiver end, to
eliminate any RF on the DC line. The benefits of mounting the pre-amp at the
masthead will be more noticeable above 108 MHz.
Bipolar transistor pre-amplifiers are usually unsuitable for serious DX
reception, because of overload and cross-modulation problems. As a general rule,
GaAsfet or Mosfet pre-amplifiers are the best option for weak signal reception.
While GaAsfet pre-amplifiers feature very low noise figures, most TV and FM
DXers will not really benefit. This is because the terrestrial noise floor is
usually no lower than 2dB from 45-108 MHz. For this reason, Mosfet amps are more
practical. Also, GaAsfet amps are very prone to static charges, which can
destroy the device. Mosfets, on the other hand, feature protection diodes on the
input, hence the device is fairly resistant to static charges.
I have built three BF981 pre-amps. One is for 45-70 MHz band 1 TV, 85-108 MHz
band 2 FM, 170-230 MHz band 3 TV. The circuit is based around the Philips BF981
Mosfet transistor, which achieves a noise figure as low as 0.7dB at 200 MHz in
an optimized circuit. My version features a more modest 1.5 dB noise figure, and
a typical gain of 20dB.
The Philips data sheets give typical noise figures for the BF981 as 0.7 dB at
200 MHz and 0.6 dB at 600 MHz. Curves are provided for determining the source
admittance necessary to obtain these optimum noise figures. This type of
performance can only be obtained with a dedicated pre-amp tuned to a single
frequency, for example, 144 MHz. Since minimum noise figure is dependant on a
high L / low C ratio, any tunable pre-amp covering a 20 MHz bandwidth will
likely have a 1.5 dB noise figure at best.
Circuit description of
BF981 pre-amplifier
The 75 ohm RF input to gate 1 of Tr1, is tuned by L1 and D1. Tr1's drain is
connected to a series tuned circuit consisting of L2 and D2. D1 and D2 are BB809
varicap diodes. For band 1 operation, L1 and L2 set the basic frequency, the
bandwidth being adjustable from 2 MHz to over 6 MHz by means of the two 25K
linear potentiometers, which adjust the bias applied to D1 and D2 (a bandwidth
of less than 2 MHz might lead to instability). For band II and III operation,
only use one 25K linear tuning potentiometer, since the lower Q (selectivity
factor) enables a single 25K potentiometer for tuning D1 and D2. This is
especially important on the 88-108 MHz FM version.
Coils L1 and L2 are wound close spaced (wire diameter) using 0.5-1mm
(19-22swg) copper wire, eg, coaxial cable inner conductor. Inside diameter is
0.25 inch. L2 is tapped a third of the way down from the LT supply end to
provide a reasonable 75 ohm match at the output.
Always try to maintain a high L/C ratio. For example, on the FM version, the
maximum coverage should be 108 MHz MHz, with minimum capacitance applied from D1
and D2.
45-70 MHz: L1 (13 turns), L2 (12 turns). The value of Tr1's source resistor should be adjusted to provide a current
drain of about 10-12mA. The resistance may be anywhere between approximately
18-50 ohms. The D (drain) voltage should be approximately 10v, while the gate 2
voltage should be around 4v.
C1 is a 1-10pF trimmer: adjust for maximum output and minimum noise figure. I
found the optimum setting was around 2pF (trimmer plates nearly unmeshed). It is
very important that C1 is soldered directly on to the input connection pin. The
lead length between C1 and G1 should be short as practicable.
L1 wire should be at least 0.5 mm diameter. I used 0.7 mm diameter coax cable
inner conductor to reduce losses in the coil. Make sure L1 has a good connection
to the copper board ground plane (earth).
Use 1/4 watt carbon resistors, and as in all VHF/UHF RF construction work,
keep lead lengths to a practical minimum. The source resistor lead length should
be as short as possible.
FB (ferrite bead) is simply slipped over the BF981 drain and G2 leads. This
helps to suppress pre-amp oscillation. Ferrite beads can sometimes be salvaged
from receiver IF transformers. Use a plastic alignment tool.
The pre-amps were built "dead-bug" style on a piece of unetched double-sided
circuit board material. The copper foil acts as a ground plane, and all ground
connections are made directly to it. See an example of typical low noise
GaAsfet pre-amp construction.
Anchor the double-sided copper board to the die-cast aluminum box via the
coaxial socket nuts and bolts. Position the Mosfet on the input side of the
screen, with only its drain lead going through to the output tuned circuit.
With minimum lead lengths, metal case (approximately 4.25 x 2.5 inches), and
screening between the input and output circuits, there should be no instability
or oscillation problems. Mount the components on a double-sided copper board
bolted to the case. Die-cast boxes, even though more expensive, are recommended.
The metal shield (shown in dashed lines) helps prevent self-oscillation at or
near the operating frequency. They isolate the input and output tuned circuits
of each stage, thereby preventing unwanted stray coupling.
Those happier with traditional air-spaced variable capacitors, might like to
try the Jackson C-82Y (3-20pF), deleting D1 and D2.
If the BB809 is not available, other varicap diodes can be substituted.
However, they must be VHF/UHF types, with a relatively low minimum capacitance.
1pF to 30pF is the ultimate coverage. 5pF to ~ 30pF is more realistic. Make sure
the minimum capacitance is no higher than approximately 6pF. The maximum
capacitance should be no more than around 30dB.
88-108 MHz: L1 (6 turns), L2 (7
turns).
175-230 MHz: L1 (2 turns), L2 (2 turns).
Testing and setting up
FM version: Tune to the bottom of the FM band (88 MHz) and swing the tuning potentiometer. A noise peak should be seen, at the bottom of the tuning range. Next tune to the top of the band (108 MHz) and repeat the process. The noise peak should this time be near the top. If the low peak occurs near the middle of the control's range of travel, the coils have too much inductance and should be spread out a little more. Conversely if the high frequency peak is too near the bottom end of the range, the coils should be squeezed together to increase the inductance.
C1 can be adjusted for RF input bandwidth. For example, with C1 set to 1pF, the input bandwidth will be narrow (be careful of possible oscillation problems). If C1 is set to 5pF, the input bandwidth will be relatively wide.
Alignment for a given TV channel is simple. Tune the input coil L1 (adjust the spacing) to the video carrier frequency, and the output to coil L2 to the audio carrier frequency. Peak C1 for maximum output both before and after aligning L1 and L2. Higher gain with reduced bandwidth and thus better noise performance would be achieved by aligning both L1 and L2 to the video carrier frequency, though this would result in reduced picture definition and low TV audio output.
Power supply
The writer uses a conventional 18v DC plug pack. A separate box has been
constructed, containing:
15v DC regulator (at least 17.2v DC is needed on the
input of the regulator).
Various filter capacitors.
Four output DC sockets
for feeding 15V filtered DC to all my pre-amps. For more information on power
filtering, refer to the VHF section in the ARRL handbook.
Parts list for tunable BF981 pre-amp
Resistors (1/4 watt): 180K, 68K, 100K, 470ohm, 18-50ohm.
Capacitors: .001 (1,000pF) ceramic (9).
10uF electrolytic capacitor.
C1 1-5pF small trimmer (10pF max).
BB809 VHF variable
capacitance diode (2). Similar specification varicap diodes can also be
used. Any VHF varicap diode that covers ~1-20pf will be suitable.
25K linear
potentiometer (2).
FB (ferrite bead) (2).
Philips BF981 Mosfet.
Wideband operation
If wide-band coverage is required, for example, 45-70 MHz, the varicap diodes and 100K resistors are left out. L1 is tuned to the lowest desired frequency, and L2 is tuned to the highest desired frequency. This means a 1-30pF trimmer is used for the input and output coils. Once aligned, the trimmers don't need to be re-adjusted.
Mosfets preamps designed for wide-band frequency coverage, typically have reduced gain compared to narrow-band versions.
Parts availability
BB809 varicap diodes and BF981 transistors are available from Maineline Electronics (UK).
BB809 varicap diode specifications
Vr (maximum) voltage 28 volts.
pF/V: 42pF/1.0v.
pf/V: 2.8pF/28v.
Q
ratio: 8.0
Conclusion
For more details on Mosfet and GaAsfet pre-amplifier construction techniques, consult the ARRL handbook. The 1989 edition (chapter 31 VHF radio equipment, features several pre-amp designs). ARRL handbook editions produced before 1993 seem to have more relevant information.
Links
Using the BF981 low noise VHF pre-amp for 144 MHz by Gordon Mcdonald, VK2ZAB.
Determinants of receiver sensitivity - What are the keys to better weak signal receive performance by Doug McArthur, VK3UM.
Low noise ATV pre-amps by Clint Turner KA7OEI.
Typical low noise GaAsFET VHF pre-amp design by Dave Blaschke, W5UN.
Understanding pre-amps by Paul Shuch, N6TX.
Preamplifier design by SM5BSZ.
86-108 MHz FM Preamplifier design by Brian Beezley.
References
T.Emslie, Low noise VHF pre-amplifier, Television Magazine, September 1997, Page 829.
