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VHF
AM Operation ...a
revival has started! |
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Introduction |
Many newcomers to amateur radio have
never enjoyed the pleasures of amplitude modulation (AM) operation as, over the
last 40 years, the use of AM has been in steep decline on the HF and VHF bands.
The reasons for this are manifold. Firstly, when lots of stations are active in
a crowded band the level of interference from heterodynes produced by closely
spaced carriers has to be heard to be believed. Many old timers will recall how
20m sounded on a busy weekend in the days before SSB! Secondly, AM is
inherently inefficient when compared with SSB: energy is wasted in the carrier
and the second sideband. Thirdly, SSB offers a better signal to noise ratio as
the received bandwidth is normally lower. Fourthly, the potential for TVI is
greater than with SSB as more power has to be produced for the same
communications effectiveness. Fifthly, AM can be badly affected by interference
from machines, engines and the like unless effective noise blankers are used. All
these factors, and no doubt others, led to the decline of AM and the almost
universal adoption of SSB for serious HF work and FM for local VHF use. |
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The HF AM Revival |
Despite this, there has been
something of a revival in AM interest on some HF bands where these factors have
been less critical in recent years. There are regular AM nets on 160m and 80m
and in the extended 40m band in the daytime. On 10m where there is usually
still plenty of bandwidth available there has been considerable interest in AM
operation between 29 and 29.2MHz, especially in the USA Much of this activity
is with “boat anchor” equipment (old, heavy, AM rigs from the 40s, 50s and 60s)
or with simple homebrewed, and often valved, equipment. The resulting signals
are frequently well modulated with excellent speech quality in a way that an
SSB rig could never hope to emulate. The ability to dust off, restore and
operate inexpensive, older, amateur or ex-commercial equipment is one of the
factors that have resulted in this upsurge in interest. Another factor is the
ability to build simple, homemade AM rigs that can be tested with the most
basic of test equipment. |
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The VHF AM revival |
With multimode, all-band, rigs like the FT817 and the IC706 family now in common use VHF AM operation is again possible for those wishing to “give it a go”. There is also a steady supply of surplus ex-PMR AM rigs still coming onto the market which, by the addition of a few crystals, offer a low cost and reliable entry route. To encourage VHF AM operation a
Yahoo Group called “VHFam” has now been formed to act as a focus. This group
was formed in January 2005 and its membership has been growing steadily since
then. Membership is open to all interested in VHF AM operation and listening.
Details of the group are at the end of this article. The group is a means to
agree skeds, exchange ideas on equipment designs and to reminisce about AM
operation years ago. It is particularly hoped that the revival in AM will
encourage beginners to try their hand at simple VHF equipment construction. |
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Recommended Frequencies |
One of the first objectives was to agree “centres of activity” for AM operation. AM is clearly never going to be a big thing on VHF or HF again so there needs to be a way of helping people who do want to operate the mode to find each other. After discussion amongst interested parties and with members of the RSGB Spectrum Committee the following have frequencies been agreed as centres for AM operation. 6m 50.57MHz These are recommended
centres of AM activity, not necessarily where all AM operation should
occur. Being in the all-modes sections,
other users such as SSTV have as much right as AM operators to their use. In
the unlikely event of lots of activity AM users would be expected to spread out
across the all-modes section. Indeed there is absolutely nothing to prevent
operation in the way common in the 1960s where people who were “rockbound” on a
crystal frequency tuned “low to high” across the band to find others on other
frequencies. |
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Homebrewing Simple QRP AM Equipment |
One thing this minor revival encourages
is homebrewing of simple QRP rigs. Many of the circuits published some years
ago are still perfectly suitable today. The “Fredbox” (see elsewhere on this
site) is an
example of what can be done with very simple AM gear. It was named after Fred,
G8BWI, who was a frequent contact with the rig from my home in Cambridge in the mid-1970s. This small
transceiver was a pocket portable AM rig for use around town and to take on
holidays. It only produced 10mW of series-modulated AM and had a simple
super-regen receiver with an isolating RF amplifier yet it managed to sustain
regular across-town QSOs in Cambridge, several 25 mile
contacts from hilltops in Yorkshire, as well as a few 60 mile QSOs
across the sea in Devon and even one at 100 miles to France. All of these were using just a small whip
antenna. A super-regen receiver might struggle in some band conditions today in
urban areas but there are plenty of ICs So, if you fancy firing up that old
AM ex-PMR rig, rolling your own QRP AM transmitter, or even just switching to
AM on your FT817 or similar commercial rig then come and join the fun on 144.55. |
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Some AM Links |
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In the last few months there has
been something of an upsurge in interest in amplitude modulation (AM) on the
VHF bands too. A number of reprints of AM construction articles have appeared
in sister radio magazines (e.g. Practical Wireless) and more AM operation is
starting to reappear on the VHF bands. On 4m it never disappeared on 70.26MHz in
some areas but on 6m and 2m the level of AM activity dropped to almost zero
with the advent of synthesised FM equipment produced in high volumes at
increasingly attractive prices. 
around that allow a low cost and
effective superhet receiver to be built. For the transmitter a simple crystal
oscillator and multiplier with the PA stage series modulated makes for a
non-complex and reliable QRP rig.