Foundations of Amateur Radio

Over the years I've had the opportunity to operate my station with other amateurs in close proximity. From a community perspective, this is loads of fun; from a radio perspective it's been frustrating to put it mildly. For example if you're operating on say 40m and your friends are playing on 20m or 15m, if you're not careful, you'll all end up hating each other at the end of the weekend due to the harmonic nature of the amateur bands.

Imagine my surprise when Keith W6KME mentioned in passing that together with the other members of the Conejo Valley Amateur Radio Club, callsign AA6CV, managed to operate 25 stations within a 300 meter circle from each other, without commercial electricity. Oh, did I mention, the antennas have to be within the circle and they operated on 80m, 40m, 20m, 15m, 10m and 6m, no doubt there were other bands that I'm not aware of, and if that wasn't impressive enough, about a third of the space was allocated to public information. You can see their efforts at cvarc.org

Mind. Blown.

Given my less than exciting experiences to date, my first question to Keith was about inter station interference and Keith pointed me at a freely available article published in the May 2026 edition of QST magazine, titled "Station Setup Techniques to Prevent Field Day RFI" by Conrad N2YCH, which discusses a whole range of stations, including the AA6CV field day station. Very informative.

One takeaway was that this is within the realm of doable for all of us. Another takeaway was that the stations in the article all appear to be high power, which is not something I have experience with, nor the budget for.

I recalled that my local friends had done some abortive attempts at using so-called coax-stub filters. Essentially several specific lengths of coax that act as filters at particular frequencies, either to pass that frequency, so you can hear it, or to block the frequency, so you don't hear it.

If you have a roll of coax, this could be a pretty cost effective way of making some field day filters.

Somewhat tangentially, the other day I was having a conversation with a friend about technology in the world. We're both of a similar age and memories of Commodore Vic 20, MOS Technologies, bulletin boards and serial port multiplexers punctuated our discussion.

At one point my friend commented on the vast knowledge and expertise held by a respected colleague, the implication being that we did not fall into that category.

Given that we'd just spent the better part of 90 minutes discussing technology going back multiple decades and their impact in the world today, I pointed out that we too fell into that "expert" category.

It's funny, even though I've been in computing for over 40 years and written over 780 articles on the topic of amateur radio, I don't particularly think of myself as an "expert". I think I know enough to know that there's plenty I know absolutely nothing about, and lots I know enough about to know that I'm dangerous.

Imagine my surprise when I searched for "rg6 coax stub filters" and discovered a post on Hackaday, called "Coax Stub Filters Demystified". It opens up with the sentence "Unless you hold a First Degree RF Wizard rating, chances are good that coax stubs seem a bit baffling to you." and goes on to introduce some interesting observations and links to a YouTube video by "Fesz Electronics" cunningly titled "Coax Stub Filters", which includes a whole lot of detailed information, simulation and visualisation of various aspects of this phenomenon, including the impact of different types of coax, combinations of these and countless other variations, that I think is aptly captured under "RF Wizard", even if some Hackaday comments were dismissive.

It brings me back to the underlying conversation around expertise.

Everyone brings their own journey to this hobby. For some a coax stub filter is trivial, for others it's magical if not magic.

I think that given the complexity of the interaction between multiple filters as shown by Fesz Electronics in their video, it's more complex than might appear at first glance.

Now all I have to do is figure out if it's possible for me to create a set of stubs for my next shared field day. I'm thinking it would help if it was modular, so we can mix and match, but that's a challenge for next time I'm left alone near a pile of coax with a VNA, cutters, coax strippers, compression tool and a packet of F-type compression connectors.

I'm Onno VK6FLAB