One of the biggest problems facing the lower band user is making an efficient antenna that will fit in to your average garden plot. Of course you can pop down to your local emporium with a fist full of pounds, dollars, or euros, and come away with a nice shiny new antenna that promises to knock the socks off of any thing else available.
But then they would say that, they want your hard to come by money.
When considering any antenna the thing to pay attention to is its efficiency.
But what is efficiency? Obviously it's the way the antenna makes use of the RF that you send to it.
It's a bit like a bank, you want one that looks after your investment, and gives you a good return on that investment.
So you want an antenna that dose the best it can with your RF.
If you are only putting a few watts down the line, it's good to know your watts are being propagated efficiently, and aren't going to
nothing more then an expensive large resistor!
So let's look at a few basics, all the books will tell you that the antenna you should be looking at is a dipole,
The formula being
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L (feet) = ------- L (meters) = -------
F (MHz) f (MHz)
So I want to get on 80 meters and using the formula I find that my 80m ½ wave dipole is a total of 133' long and for maximum efficiency needs to be 133' high!
(Remember this is for maximum efficiency though).
Firstly my gardens only 32' x 35' and I don't think I can blag it at 133' feet either, even an inverted V would need its centre point at 122'. So it's not looking good for 80m. The next option is a trap dipole, now if I build one using traps of w3dzz I can shrink the length to 110', with 55' per side, so things are looking up at last. It's still a bit on the large side for my back yard, and would have to be dog legged a fair bit, but it's doable. And as a bonus it will work on the others up to 10m (not WARC though!).
With all my antenna designs I model the antenna before building them, this way you can tell (theoretically in an ideal world) how your antenna will perform before you comitte time and money only to find it's a bit of a lost cause.
So after modelling that perfect ½ wave 80m dipole, I found nothing surprising at all, the radiation pattern was a figure 8 off the wire, an expectable angle of radiation some where in the order 0f 28 degrees. And huge nulls at each end! So already this perfect dipole ain't so perfect as you have to make a choice over the best direction for your antenna. Do you have it East, West, or North, South? Or some where in between? Ideally 2 would be needed for perfect all round coverage, but hang on a minute I can't even get 1 in so how the hell am I going to get 2 in???
The trap dipole is looking better by the minute, but is it? Compromise now rears its ugly head, any antenna that has any for size reduction by trap or contraption won't be as good as its full sized big brother. Modelling shows distortion around the traps, with multiple lobes, and varying angles of radiation, which change dramatically when bent in to the required shape, so there's more than 1 compromise in the equation. Thanks to loss generated by the traps, the overall height, and the doglegging, this is antenna is now showing an efficiency of just over 54% compared to our pukka reference dipole. And that's pretty grim, my 5 watts isn't going to be very happy, it's going to look more like 2 watts by the time you take into account feeder loss, the loss caused by having to run through a tuner, and any other watt sapping gizmo.
So all in all it's not looking very good for getting on the low bands with any efficiency, so how about going up?
With vertical antennas you have more options when space is a consideration, there are a thousand and one commercial manufactures with all sorts of designs, some simple ¼ wave verticals, some with traps to reduce height. Some with mutually inducting stubs, and others with miles of wires attached. All will work with varying degrees of efficiency, as with the ½ wave single band dipole, a ¼ wave vertical GP is the simplest form of vertical, as we know its half of the dipole but ground mounted (usually) with the other half acting as the radial, this makes things easier all round.
On 80m that's still 66.5' and that's still a long way up, but on 40m it's a more accommodating 33'. So I purchased 3 twelve foot lengths of ali tubing that slotted together with a good 1' overlap, outside diameter from 2" down to 1.5" at the top end, this makes for a robust vertical. The bottom 2 feet was sleeved with a length of pvc plumber's tube and glued with epoxy resin in to place. A couple of mast to mast clamps hold it securely to a pipe set 5' into the ground with 3foot above. To feed it I rolled a coil 6 inches long and 3 inches wide at 6 turns per inch. I installed this into a plastic box along with a 5 way wafer switch, and a post for some radials. The vertical is attached to the coil with a length of coaxial braid, and the coil was checked for swr and tapped at the lowest point and fed to a switch position, I now had 5 switch positions and 5 taps one for each 80, 40, 20, 15, 10 meters, I added one ¼ wave radials for each band.
Initial testing was encouraging, with 40 meters proving very good, but then it should as it's a full ¼ wave. The other bands had varying degrees efficiency 15 meters proving very good, again as expected due to it being a good fraction in wave length. 80 meters proved well for inter UK working but hopeless for dx work, but this is an advantage of poor efficiency, modelling showed that due to the reduced length that the angle of radiation increased ferociously up to an angle of 75 degrees plus, reports from around the country have been very good with 57 - 59 being the average. But for further out the signal just dropped away rapidly.
As with all my projects amendments would be made.
20 meters was a right disaster the swr was ok and pre construction modelling showed a theoretical 30 degree radiation angle. In a vain attempt I added more radials, but this made no difference.
Armed with a meter some tests were carried out and rf plotted, the pattern was as I expected a good circular pattern at a good theoretical angle, but the reports showed only 3's and 4's, and a lot down on my vertical comparison antenna, a Cushcraft R6000. Now bearing in mind that the R6000 is a ½ wave trap vertical with a total height of just 19 feet loss will occur due to size reduction (physical) and loss in the traps. So why dose the R6000 get me an s9 and the other s3-4?
The truth I don't know!
I should have investigated this more but as the comparison antenna was doing a good job I didn't bother, and the forth coming build of directional for 20m, I decided to concentrate my efforts on the other bands.
In comparison the 15 & 10 meter bands are better on the home brew vertical than the R6000 being around 2 S points on average.
As a further point of comparison I built a selection of the smaller hf dipoles for 40 to 10 meters as I can fit these in.
This turned out to be a good comparison test. The dipoles were fitted at a height of 33 feet, a ½ wave at 20m but a bit below par for 40m.
When listening around the bands on the dipoles the first thing that becomes apparent is the difference in the noise level. On 40 meters the back ground noise level was always at around S7 to 8, but on the vertical only S5, and the signals were al lot clearer on the audio without the noise.
The dipoles were all erected east west. An initial call was put out on the 40 m dipole and got no reply, after about 5 minutes I changed over to the vertical and got a reply to the first call, a HB9 gave me a 55 report, I explained what I had been doing and it was decided that a comparison should take place, so I switched over the antenna switch to the vertical and then got a 58 report, it was also reported that the audio quality had improved too, as before the rx was much better. But one swallow doesn't make a summer, so comparisons are still taking place, and all results show the vertical to out perform the dipole. Fair enough the dipole isn't at its optimum height, but on the other bands (10, 15 meters) the results are the same the vertical out performs the dipoles, again the vertical is less noisy and signal reports are always better. So thumbs up for the vertical. The best test so far has been with the R6000 vs. the 20 meter dipole. I thought this a valid comparison even though the home brew vertical was hope less on 20; I still wanted to compare the dipole to something on 20. The Cushcraft sure has worked for me on 20m, a thousand plus 20m digital and SSB qso's all with fantastic reports. There for the dipole with it low loss design and at a ½ wave up should give the R6000 a run for its money.
MORE TO FOLLOW SHORTLY...........................
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