2M Interference – A New Source
Vic Black, AB6SO
We tend to think of 2M interference as arising from distant repeaters, from those hams who "kerchunk" the repeater, from ignition noise and other sources. But it's possible to have interference from sources operating on much different frequencies. This article, written by Vic Black-AB6SO, appeared in the May 1997 issue of "PARAgraphs", the newsletter of the Palo Alto Amateur Radio Association, Wally Porter, K6URO Editor.
Intermodulation distortion (IMD) occurs when we get unwanted products from mixer schemes when a weak signal outside of the receive frequency squeaks by a receiver band pass filter, mixes with a desired signal and is amplified. We see something akin to this when a high power paging transmitter up around 152 MHz mixes with a weaker 2-meter FM signal in a receiver IF chain. Now Paul Bennet-N7OCS, of McMinnville, Oregon has identified another potential source of 2-meter interference: third harmonics from portable telephones, baby room monitors and 49 MHz "walkie-talkies". As Paul states it, "The 6M Citizens' Band was specifically chosen by the FCC so that any 3rd harmonic interference would fall in the amateur 2M band and not in the nearby business/public service bands", in an e-mail message, Bennet asserts that Sony portable telephones are some of the worst offenders, but he didn't mention whether he had tested any other brands or, if so, how the tests were conducted.
Apparently, some hams are receiving strong harmonics on 2-meter simplex frequencies. These may sound like malicious jamming of the ham bands, but in reality the "jammers" don't know they're causing interference. They don't identify themselves because they aren't on the radio, as far as they are concerned. Not much can be done about it, either, since both the portable telephones and 2-meter handie-talkies are FCC Type 15 devices and must accept whatever unintentional interference they receive. (An interesting aside: it is illegal to listen to portable telephone conversations. If you hear this type of interference, promise not to listen to it).
In general, portable telephones produce very weak signals. Usually they won't cause problems. What happens, though, if the third harmonic comes up on a repeater input? if the repeater doesn't require a sub-audible PL tone, we may all hear the conversation. If a particularly strong telephone signal falls on a repeater output it may create noise on the repeater channel. I made a chart of portable telephone base and remote (handset) frequencies and their third harmonics in MHz to check for potential interference.
The chart highlights standard 2-meter FM frequencies which could be affected by the harmonics. Other frequencies on the chart might possibly cause intermod distortion depending on the construction and the EF frequencies used in the ham repeaters. Some telephone remotes have harmonics which fall directly on repeater input frequencies. This could become a problem for a repeater located on top of a tall, high occupancy building (such as an apartment building) where people use portable telephones.
This chart is based on standard 15 kHz wide channels for 2 meters. If the channels are either fewer in number and wider, or narrower but more numerous, then there are even more chances for interference. Some parts of the US (Southeast and Pacific Northwest) use a 20 kHz channel spacing on 2 meters. This allows more spectrum for out of tolerance third harmonics to interfere. The new IARU Region One standard, to be phased in over several years, is for 12.5 kHz channels in order to fit more channels into a given piece of spectrum. This is being implemented immediately in the UK. This may be the way of the future in the US also. it might provide more channels for potential third harmonic interference.
Notice that this chart assumes that the consumer products are operating exactly on frequency, at specified power, with low harmonic output and with the original antenna and power supply. A channel 6 handset operating at 49.07 (i.e. I0 kHz low) might function marginally for its intended purpose but its third harmonic would then come up on the output of a 147.210 repeater. If a consumer replaces the antenna or battery on a portable telephone in order to increase range, then the effective radiated signal will be stronger. There's room for lots of combinations and permutations to create havoc. It gets even more complicated if you look at other services which can interact to cause interference. This shouldn't be forgotten by the "Little LEO" Personal Communicator Service (PCS) industry which claims to believe it can use very sensitive satellite receivers and still share the bands with other services.
Maybe this isn't all bad, though. I believe that adversity should be viewed as an opportunity. Or, as Arthur D. Little once said, "Research serves to make building-stones out of stumbling blocks." Perhaps by pulling the frequency a bit, amplifying the third harmonic, and filtering out the fundamental it might be possible to make a good FM T-hunt transmitter from a discarded portable telephone handset. Will you be the first to try it?
Is a digital voice repeater in your future?
Bill Pasternak, WA6ITF
This article is reprinted from the September 2000 issue of WORLDRADIO.
As most of you know, I work in the television broadcasting industry, now in transition as we slowly shed our analog past and enter the world of Digital Television or DTV.
At the outset, let me say I am not necessarily referring to High Definition television. HDTV is a small part of a much bigger equation called DTV. Unfortunately the media has come to equate HDTV with DTV and seems to use the two terms interchangeably. I suspect this is based on the fact that many of those reporting on the nightly news have no technical background.
Many do not realize that the accurate information is no farther than the guy or gal behind the camera or in the control room which is connected to the transmitter and antenna bringing the image (and audio) to tens of thousands of viewers. As a result, you hear the acronyms DTV and HDTV used interchangeably even though they are really two different worlds.
What if a variation on this technology could be adapted to voice repeaters? If you can transmit four or five simultaneous TV signals in a 6 MHz-wide spectral parcel, why not four or five "voice quality'' digital signals on a standard +/- 5 kHz-wide repeater pair? Impossible you say? Well don't tell that to one "next generation" repeater-putter-upper in the Southeast who believes he can. And don't tell the mid-America proponent of a repeater designed to re-transmit packetized digital audio that he will never make it work. Both amateurs have been working independently on similar exciting world of digital two-way communications. We are about to experience a quantum technology leap that will make the hallowed change from spark to continuous wave CW or the jump from full carrier AM voice to SSB pale by comparison. It will be a revolution rather than an evolution and there ma), be little time for us to adjust to one overwhelming change before another takes its place.
The question is not if it will happen -- only when. While it's less than two months since the introduction of a restructured U.S. Amateur Radio service, the early numbers are showing at least a trend toward another cycle of growth. More growth equates with more amateurs coming to the airwaves. Most of these will come into the hobby through to relatively easy-to-obtain code free Technician class VHF-only ticket. The only question now is how long will it be before their analog voice QSO goes digital and what digital system will be used.
Repeater coordinators here in the year 2000 and beyond will have only two choices. They are going to have to accept that the digital voice revolution in Amateur Radio is here and plan for its implementation or they will disappear as those with what we broadcasters call "digital vision" come to the hobby and forces them out.
Two amateurs with dreams of digital voice repeaters many not seem very important in a world dominated by analog FM voice machines, but I again note that it only took one man, Arthur M. Gentry, W6MEP, to start the repeater revolution going in our world of Amateur Radio. And while I will not attempt to tell the nation's coordination community how to do its job, I will at least suggest that they hop on board the digital train before it and its proponents pass them by.
One more note --- mainly to save myself from having to answer a lot of individual e-mail. I know I was rather vague in referencing the two digital voice repeater projects noted above, and with good reason. Both of the people requested anonymity because they are considering applying for patents and do not want to be besieged with information requests that neither can answer at this time. They have promised to keep me updated and as I learn more, so will all of you.
And this leads us full circle to where we started. The question of whether or not there is a digital voice repeater looming in your future as an amateur? The obvious answer is yes -- with or without the cooperation of the current analog FM repeater coordinators.
Why Not Get Started In Transmitter Hunting?
Transmitter hunting is an interesting subject, and one that doesn't require a lot of equipment to get started in. Here's an article on the subject that first appeared in the May 1997 issue of the Toledo Mobile Radio Association's "TMRA Amateur Radio Beacon'', Chuck Krukowski, KB8FXJ Editor.
If you've been transmitter hunting or foxhunting before, you already know how much fun it is. But if you haven't, I'll try to introduce T-Hunting to you.
Transmitter hunting is not just a game. The experience you gain could very well be used to look for downed aircraft, find an unknown keyed transmitter or track down deliberate interference.
You probably own enough equipment to get started right now? An HT. Using an HT is a little time consuming, but it's not very hard. But once you have mastered this way of hunting, you are ready for just about anything.
Before you get started, the first thing you need to know is what direction to head towards. First of all, you'll want to find a clear area with no large structures nearby. Large structures may, if close enough, reflect the signal, giving you a bad start.
Hold your HT as close to your body as possible, but still able to see your signal meter. Slowly rotate your body while looking at your signal meter. You do not want to find the peak signal (strongest), you want to find the null (weakest) signal. Once you find the null, stop rotating. The transmitter will be behind you.
If you have a full scale reading on your S meter, you will want to remove your HT antenna. Adjust your squelch so it sometimes breaks open. Again, try the body fade by rotating. It you do not receive a signal without an antenna, use a hair pin, paper clip or some other small piece of metal, proceed using the body fade.
If you do not have a paper clip or small metal piece, you may want to move off frequency 5 khz at a time when you receive a full scale reading.
Using a beam is a very popular and effective way of hunting. Two, three and four element yagis and quads can be used.
The advantage of using a beam is it's directivity. The disadvantage is its gain. You do not want gain when DF'ing, you want directivity and this is when attenuation comes in,
There are various ways to attenuate the signal. A very popular way is using a Step Attenuator. A Step Attenuator is a metallic box used between the antenna and receiver. Inside the metal box are numerous resistors controlled by numerous toggle switches. My quantity of resistors and switches depends on how much attenuation you want.
(A schematic of a Step Attenuator is in the ARRL Handbook).
You can build a cheap little attenuator using a couple of I meg linear taper potentiometers wired in series. The potentiometers must
"Transmitter hunting is not just a game. The experience you gain could very well be used to look for downed aircraft, find an unknown keyed transmitter or track down deliberate interference.''
be shielded from each other. I built one of these in an old 1 meter T.V.I. filter in a matter of minutes. It's not the best attenuator, but it's cheap and quick for somebody just starting out.
I prefer an active attenuator. The active attenuator I use has variable attenuation. It mixes the incoming signal with a I MHz oscillator. Let's say you want to hunt on 146.565 MHz. When you use the active attenuator, you will program the receiver to either 147.565 MHz or 145.565 MHz but will actually receive 146.565 mhz.
As you control the signal of the mixing oscillator, you also control the signal strength to the receiver. If you need more attenuation, you simply program 2, 3, 4 mhz off the actual frequency you are hunting.
So, here's all you need to know to get started. Get a friend and go out and see how well you do. If you don't want to hunt, at least ride along. I am sure you'll get hooked.
GRUMMAN AMATEUR RADIO CLUB
MINUTES OF GENERAL MEETING9/20/00
By Pete, N2PYV
The meeting was called to order by Pat at 6:33 PM. All present introduced themselves.
TREASURER'S REPORT –
Finances continue to be in good shape.
REPEATER REPORT –
Gordon has been trying to get in contact with Bill, N2NFI to arrange for a visit to the Hauppauge repeater site. Planning now for Saturday 9/30/00. The Bethpage repeater is working good.
NET REPORT –
There were 6 check-ins on the Wednesday 20 Meter Net. The Sunday 40 Meter Net was good. We had a better turn out for the Thursday 2 Meter Net.
VE REPORT –
There were 3 VE's and one YL applicant. She passed Tech.
WAG REPORT –
HOUSE REPORT –
Northrop Grumman is still planning to get us out of Plant 5 sometime in 2001. The trailer will be moved sometime in the next month. There is a problem with installing our tower on the new site near Plant 14. The tower will be near the Cablevision property where there is a helicopter-landing pad. This may require us to have a light on the tower fed with an uninteruptable power supply. There is a source of power in Plant 14 but it will cost about $8,000 to install tower and connect power. This is more than the Company wants to spend. Pat has obtained a form from the FAA to be submitted to determine if the light is necessary. The Recreation Department wants a statement from us stating that there are no other amateur radio clubs that have similar facilities in the area. This might help convince the Company to spend the money to install our tower. We have had a drawing made of the baseplate necessary for the installation of the tower.
The future location of the Bethpage Repeater is also in doubt. Our attempts to obtain a place on the water tower for the antenna have failed. Other locations have been discussed but nothing has been decided.
We had an interesting tour of the labs at the Underwriters Laboratories Facility.