Slim Jim Vertical Half-Wave for 27 MHz & 28 MHz

Another one from an old magazine article. This design is for a basic folded half-wave vertical for the 11m CB band on 27MHz but it can also be adapted very easily for 28Mhz or 29Mhz. For 10m operation, try reducing the half wave section to 5.10 metres and the quarter wave section to 2.53 metres.

It really is a case of experimenting and see how you get on. It's likely the biggest challenge will be the mechanical construction rather than adjusting the VSWR.

One particular advantage of this antenna is that the quarter wave tuning section effectively puts the radiating element higher above ground level. This fact alone can have a significant effect on the performance of the antenna as it begins to get clear of buildings, electricity wires and other clutter near ground level.

A lot of novice CBer's get fixated on 'advertised gain' and different types of antennas whereas they should be concentrating on getting a decent antenna up as high as possible.

While the Slim Jim won't be as good as say a Yagi beam up on a tower, it should still perform pretty well.

Back on 10 metres again...

Things have been very quiet on the radio front for the last few years. Looking at the logbook, the last time I was really on was back in April 2005. So I have been off the air for over 3 years! I guess it was mainly due to just a lack of interest. Certainly, at the moment, I am getting interested again but the whole 'DX' thing holds no interest for me. It might be interesting to get a few 'DX' contacts from a propogation point of view but I have no interest in starting to 'chase' countries again. I know that on most of the HF bands, I have more countries worked that I could ever hope to get confirmed.

So, what to do to get back on air? Most of my aerials have blown down or suffered some damage in the last 3 years so I am basically starting from scratch again. But what aerials to put up? What bands? My first priority was just to put up something to get me back on the air again, nothing too fancy ;o)

About 20 years ago when I first got on HF, my favourite band at the time was 10 metres. It's a band that has properties of both HF and VHF propogation and I used to be very active on it. I had an old CB aerial that was given to me years ago and which I used for a small while in the mid 90's. So, that was the obvious choice to get me going again.

Looking at the lenght, it seems to be an end fed half wave i.e. approx 5 metres long with a matching coil in the base. There is no name on it but I presume most of these aerials are much the same. The only question mark I have about it is whether there is much loss in the base matching coil? I seem to remember when I changed this aerial (half wave) for a multi-band vertical (Cushcraft R-6000)(3/8 wave) a few years ago, my signal on 10 metres improved a lot?
Perhaps I was mistaken but I still have my doubts about whether this type of CB aerial is a good performer.

So, at the start of November 2008, I mounted the CB half wave on a 4 metre pole and fed it with Westflex 103 coax. Why only 4 metres??....it just so happens that I have a pole that lenght and by having it that low, it reduces the wind load on the aerial.

VSWR.......
Looking at the VSWR readings that I was getting, it would seem as if the 1:1.5 VSWR bandwidth was about 700 khz. I adjusted the lenght so that the lowest VSWR which seemed to be 1:1.2 to 1.3, was near 28.300 MHz.

28.0.....28.2.....28.4.....28.6.....28.8.....29.0
1.3.......1.3.........1.3........1.5........1.8.......2.1

This put the upper 1:1.5 VSWR limit up around 28.650 MHz and the lower one was probably down around 27.950 MHz although I had no way of confirming this. By selecting 28.300 MHz as the midpoint, it meant that I could still cover most of the SSB part of the band, the CW section is really good and the VSWR down on the CB band is still reasonable so that will be useful for listening for propogation indicators down there. Looking at the 2:1 VSWR bandwidth, it probably goes from 27.7 MHz to 29.0 MHz, around 1.3 Mhz.

Comparison...Old aerial V Sirio Half Wave
The old CB half wave looks pretty similiar to the Sirio GPS27 1/2 as seen on this website. That site suggests a minimum VSWR of 1:1.2, a 1:1.5 VSWR bandwidth of 0.75 MHz and a 1:2 VSWR bandwidth of 1.35 MHz for it's antenna. Those figures look very close to the readings that I got for my half wave.

Odd effects...
The above VSWR readings were taken with the meter built into the Kenwood TS690 transceiver. When I placed an external VSWR meter in line, I got similiar readings. The strange effect was that the VSWR now as seen by the rig was a lot lower. In fact, from 28.0 to 29.0 MHz, the VSWR stayed around 1:1.1. It was almost as if by putting the external VSWR meter in line, it was helping to match the aerial to the rig. Almost like an ATU!! Strange...

Performance...
The first good sign is that the aerial is picking up lots of noise so it must be working in some sort of fashion. There seems to be a constant S '3' noise level on AM so perhaps this isn't the quietest of radio locations. Certainly, it picks up a fair bit of noise from the PC which is about 8 metres away.


CB Bands...
Listening to the CB band, there seems to be very little activity locally. I can hear a few weak CB stations on SSB at night but due to the absense of proper callsigns, it's difficult to say how far away they are. I heard one mention that he has in Rathmore, Co.Kerry which is about 50 kms away on the far side of the Boggeragh Mountains so perhaps this aerial is not so bad after all?

Another unusual feature of the local CB band is that the churches use it to broadcast the mass service to elderly or sick people who are unable to travel to church. There seems to be several of them...

27.065 MHz...FM...S '3'...Location?
27.305 MHz...AM...S '1'...Location?
27.325 MHz...FM...S '4'...Location?
27.525 MHz...FM...S '4'...Location?
27.595 MHz...FM...S ' 6'...Location?
27.605 MHz...FM...S '7'...Location?
27.615 MHz...FM...S ' 1-2'...Location?
27.625 MHz...FM...S ' 6'...Location?
27.675 MHz...FM...S '9+40dB'.....Glounthaune, East Cork.

I have them listed here with their signal strenghts on AM as they might be useful as local signal sources. That way, if I change aerials, I can do a comparison and see if there is an improvement or not.

50 MHz...
I had a quick listen on 6 metres with this aerial and I was suprised to hear the EI beacon EI0SIX on 50.051.66 Mhz (20w & Half wave vertical)(IO63nf). This beacon is approx 180 kms to the NNE from my location here in Cork. Considering the matching coil at the bottom of the CB aerial, I would have thought that 50 MHz would have been filtered out? It doesn't seem so although as expected, the VSWR is pretty high.
The beacon is about 419 in signal strenght although it does vary slightly, almost as if there is a slow variation in tropo conditions over the 180 km path. It peaks around 519 and goes as low as 319 but is always audible.

So that's it, I'm back on the air and on 10 metres again....