Sunspot Cycle 24 is beginning!

Is your radio up to the challenge?

By: Tom Phillips - AB5ZJ 2/28/08

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Some scientists believe sunspot cycle 24 began January 2008, and will peak in late 2011 or 2012. They say it is up to a year later than had been expected. Moreover, they have issued an unprecedented forecast for this new cycle, predicting it will be 30% to 50% stronger than 23 was!

If those scientists are correct, things are REALLY going to liven up on the HF ham bands when cycle 24 reaches its peak years.

Let's face it; we've been in this low cycle 23 drought for so long now, I am sure most of us just can't wait for 24's higher signal levels to begin! I'll wager even those with high gain antennas wish for this, too. No one has been immune to cycle 23's low band conditions.

So now's the time to start thinking about what effect the approaching cycle 24 will have on our day to day ham radio activities, especially as it applies to our own radios, or the ones we might purchase in the future.

As band conditions improve, I see us spending more time in the ham shack. That means QRM will greatly increase with numerous, more closely spaced S9 signals, including those at +20, 30, or even 40 over S9.

It is a certainty that these strong signals will produce more IMD** problems both inside the receiver, and also from the transmitter you are listening to, which is another huge problem in today's Ham Radio design. (Note: there are many good articles on the internet that talk about Transmitter IMD issues you need to familiarize yourself with that's outside the scope of this article.) If you are one of those fortunate enough to own a well designed receiver having high IMD and BDR (specs, it will likely do a much better job eliminating or substantially reducing those internally generated receiver interferences. However, receivers with low IMD** specifications will not fare as well when the bands begin to fill up with lots of strong signals.

Right now you may be asking yourself…. “Is my current receiver, or the one I'm about to purchase, one of those that will not fare well, when the band really opens up, due to a low IMD** spec?”

Well, if you purchased you're radio within the past few years during cycle 23's low levels, you can't really tell how it will do in cycle 24's highs unless you discover what the receiver's IMD** specs are, or have run into problems already. If you are about to purchase a new radio, then it's doubly important to find out what its IMD** spec is. (More about this later)

Having said that, if you are a casual everyday operator, and with a preference for SSB - the IMD** interference we are talking about might not matter. You can choose the times to get on the air, or simply wait till the band dies down a little. On the other hand, if you are seriously into DX, contesting, digital modes, or weak signal work, you don't choose the time, it chooses you. Any IMD** interference produced within your receiver as a result of these very high signal levels on the band could prove to be a show stopper. In that case, you'll definitely want to know how well your receiver is designed, and if it is not that good, maybe there is a mod out there that can help improve it or maybe there's a new radio in your future.

The first thing you should do is familiarize yourself with the technical aspects of receiver IMD** and BDR.** Then look up your radios test scores published by several key independent test labs such as the ARRL, Sherwood Engineering, or technically qualified Hams who have published their own independent tests.

While receiver specs are a significant indicator of good or bad receiver design, there are two important tests to look for; the 2 kHz, 2-tone, third order dynamic range, and the 2 kHz, third order intercept. These highly regarded tests, done in CW mode, indicate how well your radio can reduce multiple strong interfering signals that produce internal IMD** interference, (similar to what is sure to happen during cycle 24) while still permitting you to effectively listen to your target signal.

In the past, the ARRL tested the IMD** with test signals (tones) spaced 5 kHz apart, a broader, less stringent test than the current test which uses signals spaced 2 kHz apart. The more stringent test is used now because many technically competent hams began seeking tighter filtering from third party vendors, or they made their own modifications to obtain better IMD** results. Eventually some radio manufactures began offering better receiver designs, thus raising the IMD** bar.

It is a credit to the technical hams among us who demanded better performance from their receivers that undoubtedly contributed to an Amateur Radio industry wide manufacturing trend toward a more robust receiver design. I suspect this type of entrepreneurial spirit will one day give us receiver performance far above what we now have, and do it for less money. Still, not all receivers are made equal, and most manufactures today do not publish the IMD** specs that would reveal just how well, or how poorly, their receiver is designed.

As your own research will show, the higher these IMD** numbers are, the better your receiver IMD** interference fighting capabilities should be. Once you look at the various receiver data, it will become clear that radios with very low IMD** test scores for a 2 kHz, 2-tone, 3^rd order dynamic range of 64 dB for example, will likely become more problematic when dealing with multiple high energy signals during the peaks of sunspot cycle 24. Radios with higher test results, graduating towards 80, should do much better. Radios with very high test results moving towards 90 or higher, should fair the best. (Note: regardless of where we are in a sunspot cycle, poorly designed, tuned and maintained transmitters on the band will also degrade reception.)

Why many manufactures don't publish IMD** specs for their radios is not clear. One possible reason is to cover up a poor or marginal receiver design, which the manufacturer feels safe launching during the low sunspot years, when band conditions won't demand much of the radio's IMD** performance. They also know that most Hams lack the sophisticated test equipment and know-how required to reveal inferior receiver design. As a result, the facts remain hidden.

Since IMD** data is not published, manufacturers often rely heavily on their multi-colored brochures, beautiful pictures and catch phrases such as, “Ultra Strong receiver front end,” or ”High performance receiver design,” in hopes you'll ignore the data's absence and buy the radio for reasons other than performance. To me, there is nothing wrong with doing a little “chest pounding” as long as it is based on fact.

A good trend developing in the ham radio market is that some new radios being introduced have actually added IMD** specs to their performance data! As cycle 24 develops, these radios, in particular, will be worth taking a good hard look at.

If you are one of those who don't usually concern themselves with specifications other than sensitivity, beware. The arrival of strong signals and heavily loaded bands will push receivers to their limits and it will be these conditions that separate the men from the boys as far as receivers are concerned. If you recently purchased your radio but wait unit cycle 24 really gets going to educate yourself about IMD**, you might find yourself owning a possible out of warranty radio that falls far short of the claims made in the manufacturer's outstanding color brochure.

We should all start insisting that manufactures publish a complete set of test data that includes all the important IMD** specs (including transmitter IMD) in their marketing literature, so we can make a better, more informed decision when purchasing.

It's worth noting, however, that lab numbers do not tell the whole story about a radios performance. Depending on your point of view and operating practices, the radio should be easy to use, sound great, and work reliably. As mentioned earlier, if you are a casual operator, like to do mostly SSB rag chew, then a radio with lower IMD numbers could do just fine. For the more intense among us, well……

I'm not usually prone to nightmares but I have a recurring one these days. In that horrible dream I see a strange menacing symbol drift slowly across my vision looking suspiciously like the number…….24.

73

AB5ZJ

**IMD = Inter-Modulation Distortion, and includes IMD DR3 and others for brevity of writing.

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