I made one of those 2m Slim Jims many years ago, using aluminium welding rod (as it was to hand) and it worked very well as a fixed antenna until a gale brought it down. It also worked on 70cm for local contacts.
On the topic, I have a three band (6/2/70cm) mobile antenna which was sitting idle because I got fed up with having to remove it before going into multi-storey car parks. I recently found I had a right-angle PL259 adapter so I tried the mobile antenna on the back of the FT817 and it works a treat, though I think I need a counterpoise for 6m. It will go on the rucksack crampon holder but may cause comment as it whips about above my head! I hope to try it out on a summit soon, the 6dB of (claimed) gain on 2m and 8dB on 70cm will probably outweigh the awkwardness of its 1.5m length.
It’s a common problem because a lot of what happens is not intuitive. If you tell someone the analogy that cable is like water pipes they don’t have an issue. i.e. a 1m diameter pipe can carry much more water flow than 1cm diameter pipe and so a thick cable can carry more current than a thin cable. That all makes sense and easily understood. As soon as you get to AC then you get effects that are not intuitive. There are capacitive and inductive effects that really have to be explained so you understand why things are the way they are.
The fact that a piece of coax with an outer braid and inner conductor has three conductors at RF is very difficult to accept. Likewise I can have a certain length of coax that is a DC open circuit but is complete short circuit at some frequencies is also very difficult to accept at first. Or that antenna can be a DC short but 50Ohm at the frequency it’s designed for. Then there are end-effect capacitances and mutual couplings that effect calculated lengths. All these things make understanding antennas a real problem if you haven’t done a lot of studying of AC circuit theory, transmission lines and distributed components. But you can learn what to look for and apply rules-of-thumb to condense the fine details.
The other problem is that there is the “it’s on the internet so it must be correct” problem. It happens that I made J-pole years back and it would not match. I checked and re-checked my lengths against the design and they were the same. Was it ok on 147MHz not 145MHz? No, it was on around 130MHz. It turns out the design had misprints. Checking that the stub in a J pole is 1/4wave with the maths showed it was wrong. I dug out another design online, check the published lengths against the formulas and it was right. It required a mm or two of trimming and was blob on. Just 'coz it’s on the net doesn’t mean it’s accurate. And just because it’s on the net means it will be copied to a billion other websites and the error propagates.
A J pole is an endfed 1/2λ which is a very high impedance with a 1/4λ stub to bring the high impedance down to low impedance. They only work when they are decoupled from the feedline. That’s what the choke does and it’s critical that the choke has enough choking impedance at the frequency in use to decouple the feedline and that the choke’s self resonant frequency is below the frequency you will use.
If you get a good match on your design but performance seems rubbish compared to you commercial G pole then it will be because the choke isn’t choking enough. It tends to spoil the radiation pattern. That can easily be because you used a different make of RG58 where the VF or conductivity or insulation is slightly different. These things have an effect. The Spectrum design works because the guys running the company understand these things and have measured their design and adjusted it to be correct. I’d bet they also measure a length off every reel of coax they use to check the specs are what they expect.
You can measure all these things with a £50 pocket VNA and something like an MFJ 259B. I paid full price for my MFJ259B and wondered if it would be a worthwhile purchase. First time I used it I was delighted how trivial it made getting an HF antenna “right”. Paid for itself in a few months by saving me the one thing I don’t have much off: free time. ( Soon to change )
So assuming the design dimensions are correct and the design is not unduly intolerant of cable changes then you can make things that work. But it does require a bit of patience and experience and a few special tools. You don’t have to make many antennas over purchasing them before you get the cost of a VNA etc. back.
As Scotty said “ye cannae change the laws of physics captain.” You can’t and knowing just a few things makes making repeatable designs feasible.
Ah, but it’s a long story which I won’t tell here. It is modified for 70cm with a single feed, but I rarely carry the 70cm elements nowadays as there is little activity on the band in Scotland. No doubt you will tell me otherwise.
I think my nanoVNA is one of the best uses for £50 I have had in the hobby. As Andy says it makes cutting an antenna to size easy. I also use it for testing filters and amplifiers which makes homebrewing much more fun.
Another great use of £50 was my tarp, but that’s another thread.
Also Port Charlotte single malt whisky, but that’s another hobby.
There would be if people bothered to come on - but, I guess that won’t happen - 70cm used to be one of my two favourite bands (the other being 15m, the gentleman’s band)
“If do not find, then did you seek?” Quote from the little green guy with pointy ears!
Trouble with 70cm is everyone is very resistant to using it, its been like that for years, originally it was “I don’t have a radio for that band”, now I often hear no antenna as an excuse.
Then everyone just says the same thing about lack of activity, 70cm came alive last week during the spell of high pressure over the North Sea, plenty of FT-8, MS, SSB activity.
I have taken part in the RSGB activity contests on the band and on 2.5 watts have done just as well as I have done on 2M.
You don’t need a big antenna, even the yagis are manageable
When the “B” licence first came out the lowest frequency band allowed to it was 70cm, which at that time (1964) was very much an experimenters band. Gradually the new G8s got their stations working and activity crept up, and after a couple of years, believe it or not, there was more activity on 70cms than there was on 2m. Then in 1968 the G8s were allowed to use 2m. It being easier to get going on 2m than 70cm, gradually 70cm declined and 2m was revived as the primary VHF band. I think that more recently we have seen the same thing happen again: the Morse code test was dropped, the “B” licence was merged with the “A” licence, and HF being easier than VHF, activity migrated LF, where the challenges are different.
It takes the appearance of something that makes it easier to make contacts to get activity high on V/UHF: a tropo or Es opening, a contest, a big aurora - and sunspot maximum should bring some of those soon! The pity of it is that tropo lasts longer and is more effective on 70cm, but fewer people now have the beams to take advantage of it - plenty of underused white sticks but fewer long yagis, and you can buy a decent dual band FM rig for a lot less than a rotator!
I think its worth considering completing the set and getting the TinySA. I’ve used mine quite a bit from wandering around tracking down the latest noise source, checking harmonics and also can be used as a spot frequency generator. And takes up a lot less room than some enormous surplus HP thing. Same caveats as the nanoVNA - you’re prodding around on a tiny resistive screen and you always have to question the calibration, but useful none the less.
Hi Ian,
Many thanks for the explanation on making a flower pot antenna using RG-174 coaxial cable.
I am plannig to replicate this antenna design using RG-316 coax, because PTFE inner is more resistant to bending, etc…quite advisabe when using it in SOTA activations, backpaking, etc.
When you write:
Where the coax continues out of the topside end of the coil measure the coax to 56 cm
Moises, I would make one out of RG174 to start with just to prove the design.
RG-316 is made from different materials and has different characteristics despite also being a thin 50Ohm cable. The lengths Ian used and number of turns etc. will probably be different.
Would second that
The RG174 one I made has been very robust in use as the inner is so flexible
My dimensions for reference
Inner above the point where the braid outer is cut back : 445mm
Below that point to top turn of the choke coil 460mm
Choke 9 turns close spaced on 20mm OD plastic pipe
It resonates at 145.5 with SWR 1.05:1 when up on the pole
Cheers
Rick
Edit : PS
I think the hardest part of making a flowerpot is that the choke needs to be self-resonant just below the operating frequency. A randomly designed choke won’t do as the impedance is too low to make a sharp electrical cutoff of the lower element. I experimented at length to get it right using the vna to measure the resonance of the choke in isolation from an antenna.
I have recently been using a 2m slim Jim from Spectrum but on a recent activation I noticed that the SWR was over 1:2 which probably explained why I was struggling to get contacts on 2m. I was using it with a SOTA carbon fibre pole and, even though the slim Jim was angled away from the pole, I wonder whether the carbon fibre was de-tuning it? I wonder if anyone else has had a similar experience?
Yes, as Joe states, the carbon pole de-tunes it pretty bad. You need to use a fibreglass pole, SOTAbeams do a nice 4m light one. My SWR has always sat quite high on the slim-G, around 1:8 (2:0 on occasions), but this has never bothered me, and it’s always worked well.
I use a carbon fibre sotabeams carbon-6 frequently with the slim g, not noticed any issues and usually minimal 2m QSOs is more the number of listeners than my antenna.
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