80 Chuffin’ Metres

I’ve often poked fun at those who operate on 80m – it’s long been regarded as the sanctuary for Hams at the, ahem, top end of the age spectrum to gather for a club/local QSO and complain about their pills and prostates.

This time of year, the higher HF bands tend to close pretty early, about 5pm, so this leaves slim-pickings on <7MHz for those who are otherwise engaged in a real job.  If you’ve just got your Amateur Radio “ticket” and can only string-up a short Dipole or End-Fed, the bands probably seem full of white-noise most of the time… except for the weekend when woodwork squeaks and out come the freaks contesters…

But, don’t despair – there is some life on 3.5MHZ until well into the small hours if you’re able to get an aerial up that isn’t too lossy.  My garden is 50x20ft so even a flat-top Dipole for 40m is out of the question, but I managed to fold the wires back to make a Linear-Loaded Dipole which worked for 4 years or so.  After some experimenting, I added some larger plastic spacers and Dipole wires for some higher bands – that got me on several bands “ATU-free” :)

Being the typical Ham that I am, and never satisfied with what I’ve got – In March of this year, I decided to expand the concept of a Linear-Loaded Dipole and construct what is often referred to as a Cobra Antenna.  As far as aerials go, it’s about as simple as it gets: Some balanced feeder and as much wire as you can get in a folded/snaked arrangement.

Technically, what I have is almost an 80m Doublet – there’s about 18m of wire on each “leg” and I’ve about 20ft of 300-ohm slotted feeder which comes into the Shack and through an FT240-31 choke and then about 30cm of coax to the back of the MFJ-847 tuner.  As an all-round HF aerial, even at just 15ft up, it works really well and isn’t too unsightly – not being a Dipole, it doesn’t require a balun at the centre so isn’t prone to drooping in the centre.  This also means that the end supports don’t have to be too sturdy.

80m Linear-Loaded Doublet, aka: Cobra Antenna at M0PZT

The Doublet is often overlooked by many new Hams because they believe that a coax-fed HF aerial is the way to go – Sure, a resonant Dipole is efficient and can be plugged directly into your radio’s “aerial” socket but what about if you want to use other bands?  Coax is a very lossy feeder when used this way – the correct place for a tuner (or matching device) is at the end of the coax where the feedpoint is.  Coax should be regarded as an extension of your aerial socket – This is why the likes of SGC make water-proof “smart” tuners: Boxes that are fed with coax and that also require a DC supply so that, when they sense RF, they automatically match the aerial to the frequency you’re transmitting on.

Balanced/slotted-feeder, aka: “ladder-line”, is only marginally lossy on not just long runs, but also when used with an ATU across different frequencies.  However, there is a trade-off!  Due to it being a “balanced” feeder, it’s not as robust as coax in terms of where you can route it or how near (metal) objects it can be.  Simply put, if you upset the balance, problems will arise.

FT240-31 ChokeSome enhanced bedtime-reading can be found on W8JI’s site here.  You also have to be mindful of the length vs frequency in that it may be hard (or impossible) to get a good match on some bands due to the length of the ladder-line.  That said, those are minor inconveniences when you consider that, no matter what band you operate with your Doublet, most of the power will end up at the aerial.

The FT240-31 that I mentioned earlier is what’s known as a “ferrite toroid”, looks like a doughnut and can be used with several turns of coax to form a simple balanced-to-unbalanced device, aka: Balun.

Baluns are not complicated, providing that you select the correct ferrite material and number of turns for the job required: Ensure that you factor-in the intended frequency range and power-handling.  The most interesting part of the build process is winding the thing and this is certainly something that takes practice.

The picture here shows 8 turns of RG58 coax with a PL259 at one end (to the ATU) – the other goes to a set of 4mm terminals on the outside wall of my Shack – this is where the 300-ohm feeder connects to.

Some more information on my Shack Plumbing can be found on this blog entry.

As for performance – the map below shows total QSOs made across all HF bands since March 2016:

QSOs by M0PZT with 80m Doublet

Here’s those made soley on the 80m band and mostly during the latter part of this year – Nothing too special but certainly makes winter operating more enjoyable as there’s an extra (and fairly large) band to have some fun on.  Most of these were on CW, and a few on PSK31/63.

QSOs made on 80m by M0PZT with 80m Doublet