Once you have discovered that AM (Amplitude Modulation) is not only still used in the hobby, but is also a lot of fun for casual ragchewing you will probably want to get in on the fun.
As a child I cut my teeth listening to AM Amateur Radio QSO's and SWL broadcast on my old Hammarlund SP-200 receiver first on my Father's lap and then on my own a few years later when he gave it to me for my 6th birthday in 1964. Later I also inherited a HQ-120X. My Father was in the Army Signal Corp. during WWII, he came out a Technical Sargent, 3 strips above, 2 below and a T in the middle, he knew these Hammarlund receivers inside out, but he was not a Radioman, he was a Telephone man, he worked at Western Electric prior to WWII and he applied his expertise in land line and other technologies during the war, he even climbed poles stringing cloths line as he liked to say.
The '101 series equipment was the station that I wanted and could not afford as a teenager. Many of my friends that were a bit older had these and I wanted the same thing. I keep looking at the old Hammerlund SP-200 and HQ-120X sitting in the corner, is a surplus AM broadcast transmitter for a 160m next?
All is not lost, then again, I did acquire my Collins dream amplifier that doesn't leave much room in the house for a commercial AM transmitter, a fellow down the road has two Collins AM broadcast transmitters in his house and my wife thought that I was bad until she seen what Phil had going, he changes transmitters to change frequencies. We NJ Amateurs do things big... hi hi
The Collins 204F-1 2.5Kw amplifier pictured below is made for AM, which is saying a lot respect to any HF amplifier! This unit is the AMer's dream shack warmer, hell, entire house warmer!! Click here to read my story of my finding this piece of Collins history and about my long time coming aquisition of the Collins 204F-1 as published in DX-Magazine.
Should you have some old classic pieces of AM Amateur gear sitting around collecting dust or just came into possesion of some you will find it fun and challenging to get on AM with, but there is nothing like listening to AM on a vacum tube receiver.
However your recently purchased new or used all-mode solid state transceiver may not even have AM as a selectable mode. If it does, believe me when I tell you that the user manual recommendations for AM operation will usually leave a lot to be desired toward a good sounding signal. If you have an older hybrid or tube radio then the sound will be better achieved.
An all solid state rig with low level AM modulation can be made to sound really well on transmits and with a 6Khz or wider filfter on recieve they can be great for AM reception as well. However, these radios were designed for SSB, to make them sound good on AM they need to be properly setup, this takes a bit of doing in most cases. Many SSB rigs that provide AM are doing so with a full Carrier and USB on transmit rather than true DSB (Double Sideband) AM signal. In addition they use the SSB filter on receive and most do not even provide an option these days for an AM filter. Some do have a 6khz ceramic AM filter, but no provision for a better crystal filter from the manufacturer. My old Hammarlund SP-200 and HQ-120X receivers can be opened from .1 to 16Khz, now that is impressive sounding. The Kenwood TS-450 I own can also be selected for a wide receive bandwidth using the FM filter selection. The TS-930S with the AM filter installed can also be tuned for wide (6Khz) to narrow bandwidth using the slope tuning as can other Kenwood radios.
For instance, the TS-930S I bought in the early 80's new, it has the optional AM filter for receive, but on transmit it is USB and full carrier. If you adjust it accroding to the owners manual it has no audio level (60% modulation ?) and the carrier falls back with modulation. I if you overcome the carrier with too much audio you will sound like SSB to all using AM receivers to hear your transmission. However, if you adjust the carrier level to about 10 watts or less then with modulation you will get a nice 100% plus modulated signal with a nice upward swing. You need to be careful not to overmodulate or you will sound like an SSB signal.
My Kenwood TS-450S and Yaesu FT-890AT both have only 6khz ceramic filters, thus not as good on recieve as the TS-930, however they are much easier to use on transmit as they do not overdrive into SSB as easily and you can use more carrier power, say 25 watts. What you want to see is your AM carrier swing upward on modulation peaks rather than fall backward. A rule of thumb is that you can normally adjust your RF carrier to read one quarter (25%) of what your SSB output is on the your metering device (external Average or peak reading watt meter or monitor scope is preferred over internal metering). However this is not always true, the TS-930 is a prime example. Never adjust your exciter carrier level to great than 25% of your SSB power, you will have NO audio level to speak of in so doing. Also, if you do have a calibarted AM modulation meter, observe that you do not exceed 100% modulation much, better yet on an osciloscope, observe that you do NOT flatop.
It is obvious however, that to have a real AM signal for use on 40m, 75m and 160m you will need to drive an external amplifier as less than 50 watts of carrier, many times less than 100 watts of carrier does not make on those bands during evening hours. On 10 or 6 meters however 10 to 25 watts of carrier will work the world on AM.
For those that have older hybrid's, such as the Yaesu FT-101 series etc. that have AM mode of operation, you should not exceed 25-30 watts of carrier power as a rule. For those that have hybrids such as Kenwood TS-520, 820, 530, 830 etc. and even older Swan and other rigs that do not have AM as a selectable mode, you can modify the rig to insert a carrier with USB on transmit and remove hetrodyne insertion on receive. I did this on rigs years ago with great sounding success, the Swan 350 makes a great sounding AM transmitter !
I personally like setting up my choice of exciter to drive one of my amplifiers for about 100 watts of carrier power and modulate to 100% modulation. This yields a 400 watt pep signal which is a really good sounding AM signal, depending on your microphone, audio circuit and of course your speaking voice characteristics... hi hi
You must also take note that modern amateur amplifiers are NOT designed for AM operation. They are all basically designed for CW/SSB light duty cycle operation and AM is NOT light duty cycle operation. I can easily operate my older SB-220 on AM at 100 watts or higher carrier, the same goes for many other older amplifiers, however many of today's new amplifiers will not hold up well as the components are marginal for an AM duty cycle, AM will make your amplifer work more than twice as hard so plan on 300 watts (75w carrier with 100% modualtion) PEP for a 1kw input amplifier as the max and 600 watts (150 watt cariier and 100% modulation) pep for a 2kw input amplifier as a good rule of thumb. Remember that those tubes or worst yet, transistors in you amplifier, not mention power transformers and other components are pretty darn costly !
Technically, attaining 100% amplitude modulation of an rf carrier with a sine wave requires a modulating power equal to one-half of the rf carrier power. Under this condition, the average power of the modulated carrier is equal to 1.5 times the average unmodulated carrier power. The added power is divided equally between the upper and lower sidebands. During the peaks of 100% modulation, the amplitude of the carrier is doubled. This will cause the instantaneous peak power to be four times the instantaneous unmodulated peak power P = E2/R.
When voice modulation is employed, only the highest amplitude peaks can be allowed to modulate the carrier 100%. Since many speech components do not modulate the carrier 100%, the average power required for voice modulation is less than that required for modulation with a sine wave. Voice peaks usually modulate a carrier 100% when the modulation increases the average carrier output power 25% over its normal value.
I believe in fully monitoring my transmitted signals to make sure they are clean, especailly when running AM, I monitor my RF power output on both external pep reading watt meters and my transmitted waveforms on Kenwood SM-220 series and Yaesu FV-100 series station montiors and a general purpose 20Mhz scope. At times I also bring into play my Motorola R2002D service monitor and its spectrum analyzer when I feel I may have a problem.
The osciloscope is a wonderful peiece of test equipment to have in an Amateur station, from setting up your transmitter and amplifier and monitoring for signal purity on transmit to RTTY tuning and in some cases pan display. The dedicated station monitor scopes are even better in most cases. The SM-220 station monitor or other make or a general purpose scope allows for an accurate visual of the quality of your transmitted signal, be it CW, SSB, AM, RTTY or whatever. With a general purpose oscilloscope and a 1000:1 probe properly calibrated, when you measure peak voltage across a 50-ohm termination a reading of 400v peak equates 1600w pep if you are attemping to measure maximum power on a scope, see the graphic below. In addition the SM-220 provides a number of features for specific transmitter testing what with a built in two-tone generator and the capabitiy of trapozoid transmitter and amplifier display. It is much better to set up your amplifier with a two-tone gernerator for max output and not exceed that point on the scope.
For an interesting presentation on determing percent modulation on AM using an osciloscopeclick here.
For those interested in a more technical presentation on AM can visit the following U.S. Navy site by clicking here.
What does all this mean in regard to full legal limit power levels and the FCC when operating AM ? That is a complicated issue, however, it would be safe to say that most Amateurs will find it very difficult to reach the 1500w pep legal limit operating AM. This is especially true using any equipments manufactured during the last 30 years as AM is a 100% duty cycle and at 100% positive peak symmetrical modulation a carrier level of 375 watts would be required. If you can, you will find it a great test for your Amplifier, Antenna Tuner, Balun's, Antenna Traps, Coax, etc., AM and RTTY will separate the man from the boys in that regard.
For more information refer to FCC Part 97 or the ARRL Handbook or visit the pages of WA3WDR by clicking here.
Also good reading is Amplitude Modulation ("AM") using natural asymmetrical voice a joint effort by John, WA5BXO; Bob, WA3WDR; Tim, W5TOB and Don, K4KYV which can be found clicking here.
AM is a lot of fun and along with CW it is real radio to most of us in the hobby. Chasing DX and Contesting and to a certain extent, ragchewing as well on CW and SSB is great fun as are the many other pursuits on the hobby, but good old Ancient Modulation is real radio to me that does not get my wrist tired from clarifier adjustments or keying. The bands are crowded however, so be prepared for some side channel interference, especially from the sideband stations that do not even know what AM is about or those that do and don't want it around.
For an interesting reading on AM channel spacing by WB3HUZ click here.
Listen for N2CKH on AM above 29Mhz or above 7.280Mhz mostly on weekends during the day time all year round. I also visit the AM windows on both 75m and 160m in the evenings mostly in the Winter when the static crashes are lower, but I could pop up just about any time. If you are active on 6 meter AM you may find me on 50.400 or about with my old Kachina 1 or FT-680R, I have an SB-200 awaiting modification for 6 meters, so at some point my signal should be better, I never had more than 100w on 6m and have worked the entire U.S. and nearly 100 countries OCONUS, so more power will interesting, it would be nice to completer my DXCC before I move to NC, I did not want to go moon bounce on the last cycle to do so as some did, however as to AM, in my part of the country I just do not seem to find much AM activity on 6 meters these days, its not like it was when I first got on 6m in 1981, mostly all AM and CW then, now AM is rare, just like the old Kachina 1 even my Kachina 505DSP is getting old and rare.
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Entire contents Copyright © 1999-2001 by Stephen B. Hajducek, N2CKH. All Rights Reserved Worldwide.