VHF AM

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 many. 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 and FT8 for serious HF work and FM for local VHF use.

HF AM Operation

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 on the 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.

VHF AM Revival

In the last few years there has been something of a small upsurge in interest in amplitude modulation (AM) on the VHF bands too. A number of reprints of AM construction articles have appeared in 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.

With multi-mode, all-band, rigs like the FT817 now in common use, VHF AM operation is again possible for those wishing to “give it a go”. There was a steady supply of surplus ex-PMR AM rigs coming onto the market in the UK which, offered a low cost and reliable entry route.

To encourage VHF AM operation a Yahoo Group called “VHFam” was formed in 2005 to act as a focus. It recently moved to groups.io. Its membership has been growing steadily. Membership is open to all interested in VHF AM operation and listening. Details of the group are at the end of this page. 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.

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 needed to be a way of helping people who do want to operate the mode to find each other. After discussion amongst interested parties the following frequencies were agreed as centres for AM operation.

Sadly the RSGB band plan only notes 144.55MHz as an AM centre of activity, rather begrudgingly in the footnotes. It is high time our national society put this frequency properly in the published band plans. Just DO IT please RSGB!!

4m 70.26MHz

2m 144.55MHz

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.

Homebrew 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 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.

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.

To subscribe to the VHFam group e-mail to: VHFam+subscribe@groups.io