Radio

Well, I had a full wave loop for 20 Metres, which I tried to convert into a folded dipole (now this is proof that one must think before one acts) which was a failure as there wasn’t enough wire strangely enough, and to top it off after the strong winds in Melbourne the other night caused some damage to that failed antenna.

Now generally for a dipole we work on the theory that the full length of the dipole is ½λ so for example for the 20 meter band we could go for the following formula:

(300 ÷ 14.175) ÷ 2

where 14.175 is the centre frequency I’m trying to work with.

So from this we get the value of:

10.582 metres

As I’m working with dipoles we can simplify the formula above to just 150 ÷ λ where lamda is the frequency we’re working with. I’m wanting to get good coverage over the 20 meter band so I’m using the frequency of 14.175.

So for a normal run of the mill dipole our two “legs” of the dipole would be:

5.291 metres

Now a folded dipole has the characteristic of offering a better match over a wider band. Meaning if all goes well the folded dipole will work over the entire 20 metre band without having to use a tuner.

Though using a tuner and balanced feed line it is said that one can use the folded dipole on other bands where the half wavelength is at the lowest frequency you’re operating on.

Now for my folded dipole, the full length of it for ease of construction was 10.6 Metres, and the distance between the top of the folded dipole and the bottom (I’m sure there is a word for this but for the life of me I can’t think of it.) was .6 Metres.

Now the feed impedance of the folded dipole is somewhat higher than that of your standard dipole (which is around 72Ω) , and is close to 300Ω so instead of the 1:1 Balun that a standard dipole would use a 4:1 Balun to try and bring the impedance as close to 72Ω as we can.

Anyway, after building this, and putting it up in the air, I’ve checked the swr of the antenna, and it appears to be 1.5:1 across most of the 20 metre band. So I need to tune the antenna up a little, as it is just a touch too long. But it works very well compared to the full wave loop I had used previously, and much better than my first attempt of a folded dipole.

Tags: antennas by Ivy
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Well I wasn’t happy with the full wave loop for 20 metres.  While it was quiet, I couldn’t hear much either, and also I was proving difficult to match.  I may not of  thought things through properly when measuring it and may of left it a bit too short.

Anyway I’ve had some luck with folded dipoles previously (on 40 Metres and 80 Metres.) So I decided to build one for 20 Metres.

So far so good, not a perfect match it’s about 1.5:1 across the band.  But that’s not too bad.  I might see what I can do to improve the match.

So far so good. Just listening to someone up on the Birdsville Track.

~Ivy

Tags: antennas by Ivy
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Well set up an inverted V dipole for 80 Meters.  And so far so good. It has a 1:1 swr at 3.5 MHz, and about 1.5:1 at 3.6 MHz.

So as I don’t do much in the way of CW, I could shorten the antenna a little, so to get it closer to 1:1 around the area in which I tend to talk on.  Which is around 3.59 MHz for a little group that I like to touch base with.

Also with the help of VK3GNU, who is much better at getting ropes up into trees than I.  We set up a full wave loop for 20 Metres.

Still thinking of what else to build.  One of the things we’re considering is a beam for 20 Meters.  How many elements, and do we buld traps to make it shorter. Who knows but it will be a lot of fun.

Tags: antennas by Ivy
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Finally got the 40m dipole up at the new QTH. Today I’m going to call cq and see what I get.

Tags: antennas by Ivy
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After having some luck with simply putting scrap metal into the air as a vertical antenna at 14MHz, we decided to try and do things properly. A trip to the local hardware store has proven quite useful for sourcing antenna elements. 3 metre lengths of 10mm aluminium tubing came out to only AU$8 or so. Of course, it aluminium tubing can be sourced more cheaply (especially as scrap), but this was more convenient for simple experiments.

It seemed like the biggest drawback to our previous verticals was the height above ground. Our copper gas pipe experiment had the base of the antenna only 3 metres above the ground, with the radials lazily laying on nearby objects.

This time, we decided to utilise our EMDRC 9 metre collapsable mast. We had decided to use PVC water piping as the mount system. Unfortunately we never bothered to collapse the mast and measure the outer diameter of the upper section, and ended up buying pipe which was too wide, and possibly too thin-walled. With a bit of persitance and trying again to get the holes to match up with the screws (how do you properly measure straight lines on a cylinder?), we had about 5 metres of aluminium in three telescoping segments on PVC. For the radial system we used self tapping screws to attach a 5cm wide alu strap around the PVC below the vertical element, with one larger hole for attaching an SO-239 socket. The centre conductor of the SO-239 is attached to a short jump lead with a terminal plug around the mount screw at the base of the vertial element.

20m vertical test on ground

Detail of vertical element and radial mount

Mounted vertical ready to raise. Dipole mount for 80/40 below.

Raising 9metre mast with 20m vertical

It began to get dark by the time everything was constructed and roughly tuned. Turns out we had mixed up some of the radials and guy wires, which wrapped around each other at the top. Not expecting good results the first on the first try, I was surprised to find it gave little to no reactive loading at resonance, which was around 13.9MHz. The resistive load of the antenna was much higher than expected, at nearly 200 Ohms. I had read the angle of radials to vertical element can change the Zo of the system, but had no time to try at the time.

Coming back to it, and getting the radials as close to 45 degrees from vertical as possible gave good results. We were able to get the Zo down to a good-enough match of 60 Ohms across the lower section of the 20 metre amateur band. the 2:1 bandwidth was across any usable frequency for amateur use.

The 1964 ARRL Antenna Handbook gives ideas for multi-band verticals containing multiple resonant vertical elements with 4 resonant radials for each band. This PVC mounted system may lend itself well to this setup, just in time for the growing 10 metre openings.

de VK3GNU. 73

Tags: antennas, musings, tech by Hannah
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Over the past months, Ivy and I have been trying to catch those DX stations. In the current time of solar minimum, it has been hard to return to the calls with our meager 100W. And, not being able to afford stacked monobanders and a leenyer, we’ve decided to try and improve what we can, which is antennas.

The first choice was to install HF verticals for different bands. Since they are relatively easy to construct with just wire, and have a low take-off angle of radiation, we decided to go for it.

Wonky 20 metre vertical

First was a 20 metre vertical. Scrounging around for parts, we came across a sad piece of copper pipe that had been left outside for some months. After straightening it out the best we could, it measured just over 4 metres in length. My understanding of antennas isn’t all that great, but I do understand velocity factor. Copper pipe is said to have a VF of 0.86 or so. Given our poor copper was in an advanced state of oxidation, I figured the VF would be lower.

Oh well, might as well just load it up and see. After sanding off the corrosion on the bottom and attaching a strapping wire via a clamp, it seemed to have OK continuity. Then we decided to try 4 radials of scrap wire, each at 1/4 of a wavelength. The vertical was then installed on the roof with surprising results. Though we could not get the SWR down below 1.7 or so, it happened to have this dip at ~14100kHz. This antenna performed quite well, considering it cost us the grand total of naught

With the success of 20 metres, we decided to again try, this time on 40 metres. Around this time we decided to buy one of the EMDRC 9 metre aluminium masts. While it’s a tad short for the 10 metre tall 1/4 wavelength vertical at 7MHz, we came up with a clever solution to extendorise the mast. Using a 3 metre long bit of orange electrical conduit, we slid our extender into the top of the mast section. The whole thing flopped right over to one side. But hey, the radiating element can’t be too close to the metal mast anyway, else it will couple and cause a distorted radiating pattern and possibly effect impedence. The bendy conduit put the vertical about a metre and a half to one side of the mast, and the bottom of the 10 metre radiating wire ended up just above the ground. More 1/4 wavelength radials, and we were on the air. This one, though, was able to be tuned by way of changing the length. We managed to bring the SWR down to about 1.2:1 at ~7200kHz.

Results with the 40 metre vertical weren’t as drastically increased as with 20 metres. We got only slightly better (often only 1 S-unit) signals on the vertical compared to the dipole. The radiation pattern for transmit seemed to be pretty similar to the dipoles as well. It was useful as an antenna for local contacts, but we still couldn’t get those DX stations to hear us.

We’ve since given up on the verticals, in favour of a ZL-Special. There will be more to come.

73, Hannah

Tags: antennas, dx, tech by Hannah
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Tried to build a 146MHz J-Pole using wire, following some of the online guides. It seems simple enough: stick a half wavelength vertical on top of a quarter wavelength matching section. The ends of the matching section have very high impedence, while the bottom is very low. So, somewhere on that path is 50 Ohms. I must admit my ability to construct isn’t the best, so the joints and connections are probably shoddy at best. Ended up using a 150 cm jumper coax bought from a local supplier. Cut one end off and connected alligator clips, one to the braid, one to the center conductor. Made few loops of coax about 4 cm in diameter as an RF choke.  Alas, I’ve been told I have a hum in my transmission. Not having an antenna analyser for VHF, I can’t *really* tell where the 50 Ohm feed point should be, but even just a few millimeters off shouldn’t add a hum. I’m a bit baffled. I guess the best explanation is not enough RF choking, and the microphone or IF stage in the radio are picking up a bit of stray RF. Next will be to add a ferrous material to try and increase this.

Eventually, I hope to build a larger J-Pole for 14MHz. This isn’t a very conventional antenna, but will be the easiest for our situation. Having access to a large tree will allow me to put the ~15 meter tall antenna up rather high. It will have a feedpoint close to the shack, at the bottom of the tree. Best of all, it won’t require a huge amount of radials to get a bit of low-take off radiation happening. Hopefully this will work for a bit of DX on 14MHz.

Tags: antennas, musings, tech, vhf+ by Hannah
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