Evening project: Junk yard 2m/70cm "flower pot" antenna

Nearly a month back now… there was a working bee at HSBNE to clean up the bone yard and generally spruce the place up. In amongst the “junk” going to the tip was this ~1.5m fibreglass rod which I thought was too good to just cast away. It’s a good length for making a groundplane-independent dual-band antenna for the 2m and 70cm bands, and as a bonus is nice and lightweight, and flexible.

It should make a nice replacement to my aging ¼-wave 2m whip. The design I am following is this one:

http://www.g8njj.org.uk/index.php/82-homepage/90-2m-and-70cm-flowerpot-antenna

The first step here is making the balun on a ~22mm PVC former. This I plan to try and mount to a PL-259 connector I have here and will form the base of the antenna. Underneath the tape is the 9-turns of RG-58U coax.

Next step is to measure out 480mm of coax, then ring-bark it and strip the jacket and braid from the next 490mm of coax.

Having nibbled away at the coax with a pair of end-cutters… I can now wind the choke. This is one turn less than before, on the same sized former. I just carefully cut my previous section of pipe down to size with a hacksaw, drilled a new hole in the new piece and started winding.

At this point, the antenna is coming to shape… and right away I see that I’ll need a second bit of plastic or something to extend it a bit. The good news is that the mechanical support needed at this point should be minimal.

I do see one problem though, I’ll need another bit of PVC between the balun and the choke that’ll be covered in aluminium foil: I forgot that bit, so I’ll have to undo that choke, but I might as well measure out the final length of coax first as it’ll make my life easier.

So having taken apart the choke, threadded the coax through a 235mm length of PVC pipe, then re-making the choke, I made an alarming discovery… seems the whip is too flexible, and cannot support the weight of the chokes and the coax.

No matter, I have a second, heavier-duty length of fibreglass rod, that was also given to us for such projects. I’ve just got to figure out a way of making a base with the PL-259 plug integral to it. That, and the other 235mm length of PVC pipe just went missing, so I’ll put it aside for the evening given it is now past midnight and I can resume this in the morning.

Okay, so re-thinking the problem… one issue I have is securely mounting the whole lot to the PL-259 connector at the bottom. That’s always going to be the weak point, as I don’t have the luxury of making the connector integral to the antenna.

So I’ve re-thought my plans a little bit. The original rod was nice and light, and more than long enough to make a vertical sleeve dipole, which is what I originally had planned before I dove off into more complicated designs like the dual-band flower pot. Perhaps the way forward is to turn a base out of a 30mm block of wood and make it a fixed base antenna.

Going back to the original fibreglass rod, I noticed that the heat-shrink hid a small length of hose, and that could be stripped away to give me just the bare rod. This, could be jammed into a clamp-style PL-259 I had laying around, meant for RG6 coax. There’s enough room for a length of RG58 to come out of that connector alongside the fibreglass rod. I could then make a sleeve dipole by peeling the outer sheath and sliding the shield back over the coax.

This is the length of coax being prepared. I normally ignore velocity factor, but in this case I decided to factor it in, I can always strip the coax back further to make it longer.

I normally tune my 2m antennas for 145.7MHz which is mid-way between 144.1MHz and 147.3MHz. I don’t do CW so the bottom 100kHz of 2m is of no interest, and there might be some simplex stuff between 147.3 and 147.5MHz, much of that upper 500kHz is repeater output frequencies, so I’ll be listening rather than transmitting.

With that in mind, 145.7MHz gives a wavelength of 2.059m… quarter wavelength is 514.75mm. Accounting for 0.66 velocity factor for RG58/U coax, I get 339.7mm, call that 340mm. That’s the length of exposed braid seen in the above photo.

I found that this particular coax is pretty stingy on the braid, so when I peeled it back along with the foil, I found it difficult to get that to wrap around properly. Plus I had managed to accidentally cut the shield. Ahh well.

I have some adhesive aluminium tape, so I used a length of that to replace what was lost. Bonus being it adheres the coax to the fibreglass rod. Downside is it won’t take regular solder. @pelrun pointed out (on Slack) I need special flux for that, which I do not have. (Did try some flux I happened to have, but it was no use.) So workaround, use some scrap stranded wire, solder the braid to that and wind it tightly around the top of the aluminium tape and solder it.

At least at DC it checks out. I’ll try it out on the radio in a moment.

That’s the full antenna completed there. I just wrapped some tape around the bottom to secure the rod in the connector, long-term I might look at what I can make to clamp around that and better secure it, but at least it’s together now.