The spectrum from 40 to 47 MHz is used by commercial and state agencies in North America which means that there is a certain amount of equipment available for the band.
Pasternack Enterprises is one such provider of equipment and they sell a mobile antenna which covers 40 to 47 MHz.
It would seem that this is a full size quarter wave antenna and at 40 MHz would be just under 2 metres in length!
The antenna costs in the region of $60 with the associated mount costing about $50.
While it is pretty easy to build an antenna for 40 MHz, the difficulties of mechanical strength and waterproofing are always a problem. Some may considering buying a commercial antenna and this is an example of one from SIRIO.
The SIRIO GPA is a basic quarter wave ground plane antenna and there are several models available to cover various frequency ranges.
Over the years, I have collected many old magazine articles and it seems to be a waste to have them buried away in a folder. This one is for a 3/8 wave ground plane antenna for 28 MHz and hopefully it might encourage someone to experiment and build one.
The principle is pretty easy. A normal dipole in free space normally has a 72 Ohm feed point impedance. A quarter wave ground plane is half this...about 36 Ohms.
By extending the antenna from a quarter wave to a three-eight wave, the maximum current point moves from the base to further up the radiating element. This has the effect of lowering the angle of radiation which is a plus.
The longer three-eight wave antenna now has a reactive feed point (inductive) as it's too long. This is tuned out with a series capacitor and it should be able to get close to 50 Ohms.
This antenna would suit someone who can only put up a quarter wave ground plane for 10 metres and wants better performance.
The article from the March 1983 edition of Short Wave Magazine is shown below...
The long process of getting back on the air continues. I've put up antennas for 2m and 70cms in the attic of the house so at least now I have something that might be considered semi-permanent. If I go off the radio again, it's not such a big deal to get going again and the aerials won't get destroyed by the weather.
In the last week, I checked the Westflex 103 coax cable going to where the old CB vertical used to be. Using my old home brew power meter and my old Yaesu FT-290R, I established that the loss for the 20 metres or so of Westlex 103 was about 2dB at 145 MHz. This means that it should be well under 1dB at 28 MHz and that includes having a back to back SO239 connector in line. So the coaxial cable is working fine and shows no sign of water ingress.
About a fortnight ago, I purchased a CB half-wave vertical from Long Communications in Donegal. The model was a Venom which I believe may be from a company called Sigma.
Considering it was only €35 with an additional €12 for delivery, it's hard to say it is in any way expensive. In contrast to some of the verticals for the HF bands, it's incredibly cheap.
Some of the specs should be taken with a pinch of salt.
Power Handling...1000 watts.....I doubt it. 100 watts is fine but I'd have my doubts about anything close to 1000.
Gain...4.1dB.....Sounds great but it's a half-wave! 0dBd or 2.15 dBi at the most and that's if you ignore matching losses.
Frequency....26 to 28 MHz.... My tests shown below suggest that it won't be resonant anywhere near 26 MHz. Without an antenna tuner, I think 27.0 to 29.7 MHz would be part of the spectrum that the antenna could be adjusted to.
Construction and Assembly.......All the parts come in a small plastic bag which includes two U-Bolts, three screws, three washers, 3 nuts, 4 jubilee clips and a small plastic seal for the top of the antenna. All pretty obvious and the antenna can be assembled in about 10 minutes.
There are Blue marks on the aluminium sections to show how far each section should be inserted into each other.
This YouTube below from someone in the UK shows what it looks like out of the box including the Blue marks for adjustments.
One of my biggest fears was that the antenna would be really flimsy once it was constructed. To be honest, it looks fine. I can't really say the construction matches the cheap price, it seems fine.
Tuning...I mounted the antenna on a 4 metre pole and checked the VSWR with the internal meter on the Kenwood TS690. When I had the lengths adjusted to the Blue marks, the centre point seemed to be around 28.0 MHz. I then shortened the antenna so that the VSWR plot now looks like this.
Some observations and notes....
1) As I shortened the antenna the VSWR curve below the centre frequency seemed to match that above. As a result, I have inferred the plot below 28.0 MHz as I cannot transmit there.
2) The 1:1.5 VSWR bandwidth is 1.3 MHz which seems pretty good.
3) The top section of the antenna must be adjusted by 4 cms to move the centre frequency by 100 KHz. So in other words, if you wanted to move the centre frequency up by 300 KHz then you would make the antenna 12cms shorter.
4) If the length of the antenna was assembled to the Blue marks then the centre frequency was 28.0 MHz. This means the VSWR was at or below 1:1.5 from about 27.35 to 28.65 Mhz. This might be an issue for someone who wants to use the bottom end of 27 MHz as there wasn't much scope for making the antenna much longer. It should tune up fine with an antenna tuner but I'd guess most CB stations would be unlikely to have one.
5) My antenna was mounted at 4 metres above ground level. The VSWR and centre frequency may well change if the antenna was at different heights.
Performance.....Too early to say but I am hearing local Churches broadcasting at the top half of 27 MHz from about 50-60 kms away so it seems reasonable. It is certainly no worse than the old CB half wave I used to have.
Overall.......Time will tell if it survives the Winter gales but the mechanical construction seems reasonable. If I had it way above the house then I suspect it wouldn't last long. But at 4m, I would expect it to last the first winter anyway. Other than that, the performance seems as expected.
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.
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...
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....
