Stray Signals

Shakespeare and Coax Stub Filters


Foundations of Amateur Radio

Shakespeare and Coax Stub Filters

If you read it on the Internet, it must be true. but what happens if you read it and there are 700 different answers?

In my day job I search countless times a day for answers to problems. Based on my experience I can look at a list of responses to a question and tell myself what the skill-set is of the poster, they don’t know what they’re talking about, they’re guessing, they’ve got no clue, they tried it, ah, this one knows what they’re talking about.

As an aside, a company once advocated that we should use social media as a way to provide support to customers, but based on my experience, seeing the correct answer in a series of posts being voted down into oblivion and seeing the wrong answer being promoted is a fantastic example of why that won’t work, ever. Infinite monkeys with typewriters might eventually write Shakespeare, but it will take an infinite amount of time and before they succeed there will be a whole lot of rubbish.

When I started researching magnetic loop antennas several years ago I went through the same process, search for answers online. I found lots of different stories, opinions, measurements, contradictory statements and formulas. I spent some weeks reading everything I could on the subject and after a while a picture started emerging that started to explain to me how a magnetic loop antenna works. I’m no expert, my foray into this died when two ADSL modem transformers died within seconds of me hitting the PTT on my radio and I sort of lost interest. I have a magnetic loop antenna standing behind me, on loan from a friend and I use it to scan the bands. It’s compact, easy to tune and one day I’ll make more than a single contact on it.

All this to say that I’ve been investigating coax stub filters.

If you’re not familiar with the notion, you can cut a piece of coax cable to a specific length and connect it with a t-piece to your antenna feed line. If you do that, depending on the length of the coax you cut, you get interesting effects. These effects include filtering, or notching out frequencies, passing other frequencies and all in all affecting what your radio is able to receive and transmit.

If you’ve ever set up your radio with some friends nearby for a field day, a contest or a camp-out, you might be surprised to learn that even across the space of a field or a caravan park you’ll be able to hear the other station, even if they’re not on the same band. You might hear their actual voice, or more likely, you’ll hear all manner of overload sounds that essentially show up as noise blocking out the station you’re really trying to hear and work.

Of course, the reverse is also true. When you’re transmitting, your friend is hearing the same horrible gunk coming out of your radio.

One of the ways that you can manage this is to set up filters, either notch filters which reduce the strength of undesired transmissions or pass filters which only allow certain frequencies to get to your radio. Combining these will make your life much easier.

Coax stub filters are a tried and true method to achieve this and the Internet is full of expert opinion on how to exactly do this. With infinite budget and time, you can try them all out and with your trusty network analyser you can find the combination that works

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