1/4 wave GP Antenna

[halfwave]

Hi,

Just came across this 1/4 wave GP antenna on ebay
and wondered if anyone has bought one of these and
how it performed v's a tuned Dipole?

http://www.ebay.co.uk/itm/1-4-wave-Prof ... 3641.l6368

I see that the radiating element is made up of combining various pre-measured
lengths which cover small frequency ranges. Would this have a detrimental effect
on SWR as it is not tuned exactly to the specific frequency to be used?

Also there seems to be no Balun, would it require one?

[NOYB]

Well it's "professional" so it must be good!

Seriously though, I wouldn't have thought the sections would cause a problem initially, but maybe when/if the aluminum oxidises it might cause issues? I'm not an expert but wouldn't have thought that sort of antenna would give any gain (I guess their figure is dBi, not dBD).

If you buy one, please don't leave the whole weight of the feeder dangling on that poor connector like they do in their photo!

[yellowbeard]

On a ground plane, one of the functions of the radials is to prevent RF flowing down the outside of the coax, as well as providing a ground plane and setting the impedance with the angle. That particular item has the coax coming down the mounting tube which is also good. On a dipole you can take slivers off it with an emery board until the SWR is perfect, but unless you are doing this up the pole with the run of coax you are going to use, it is an approximation at best and will change a little when installed and with weathering. That said, personally I wouldn't pay that money for such a simple antenna, ground planes are easy to make and tune - mounting it securely to the pole is the hard part. I am a tight bugger though...

[radionortheast]

if built set up right should work better than a dipole, radiation is more like a squashed dome, more of the signal is going out instead down and up in a dipole, (the angle is lower) theres bigger null below the aerial.

made mine from bits of boom arm, with thiner 1cm aluminum going inside, it has no effect on swr that their made up of different bits….its the lenght of radials and angle that effect that also change the impedence, i can see the swr impedence change on my rig expert when alter the lenght change the angle. the perfect angle lenght is dependent on where the aerial is.

[Albert H]

The angle (and the lengths) of the radials affect the feedpoint impedance. We used to make these out of "Microbore" copper plumbing pipe. The advantage of this thin pipe is that it can be bent to shape really easily. It's also pretty soft, so it's really easy to cut. I used to cut the radials an inch or so too long, and the radiator would be about 2" too long.

My groundplane aerials were built on to a rectangular metal plate that had a right angled bend in the middle. The piece of metal I used was 2" × 4", so when it was bent, I ended up with a 2" square to mount the connector and the radials to. The radials would be flattened at one end, and a couple of 3mm holes drilled, to allow M3 nuts and bolts to secure the radials to the main plate. Corresponding holes had to be drilled in the horizontal part of the flange plate.

In the middle of the plate, I used to drill and punch out a centre hole to allow fitting of an "N"-type or a 50Ω BNC. The radiating element was soldered directly to the centre pin of the socket, the radials were attached (unbent, so that the thing was still easy to work on) and a yoghurt pot or similar small plastic bottle was fitted over the whole Socket rear and lower radiator end. The yoghurt pot would have a tight-fitting hole in the bottom of it and the thing would be slid down the radiator to the metal flange. I used to gaffer-tape the yoghurt pot to the flange to prevent leakage, then I'd pour in some pre-mixed epoxy resin. The stuff used to take several hours to dry, but you were left with a waterproof and mechanically robust base of the radiator.

I'd drill four holes in the metal flange plate to allow mast clamps (the "U" type) to be fitted. The coax would be connected to the socket with the appropriate plug. and the coax would go down the inside of the mast to prevent malicious people putting drawing pins in the coax or cutting it.

The final part was easy - I put the aerial on to a 3m mast in my garden, and fired 1 Watt into it at the frequency I was interested in. I'd bend the radials down progressively, watching the reflected power fall away on my SWR Bridge. When I'd got down to about 1.2 : 1, I'd trim the radiator for best match. That would get the SWR close to 1 : 1, but then final tweaking of the radials would get it exact.

The ¼-wave groundplane has no gain (over a dipole), but has three useful properties: It's easy to match accurately, it's robust and cheap to build, and it puts your energy where you want it - little of the signal goes upwards or downwards.

If you want to build a version with useful gain, the same construction method can be used and you can make a ⅜, ⅝, or ⅞-wave vertical - each with ¼-wave groundplanes. Each of these requires some kind of loading coil - mine often used to be built into the yoghurt pot!

Of course the inevitable happened, and I used a "Ski" yoghurt pot, so my Polish pal used to call it the "aerialski" or the "skyski"....

Remember - the yoghurt pot is just used to shape the epoxy resin, so can be cut off afterwards if you don't want your aerial to look silly!

[halfwave]

Thanks for all your replies.

One quick question, the antenna shown in the ebay listing looks like it only
has 3No. radials, where as most other images I have seen show 4No.
Does the amount of radials make a difference? If so which is it best to go for 3No. or 4No. ?

Albert, you stated that when tuning your antenna you progressively bent down
the radials to reduce the reflected power, does this mean that dependent upon
the frequency, the radials would have to be bent to a different angle?
As well as getting the radiating element cut to the correct resonant length.

The one on the ebay listing looks like it would give a fixed angle due to how
the radials connect to the antenna they just screw in at fixed points.

[NOYB]

The perfect ground plane would be a sheet of metal - presumably a downwards cone for this antenna. The theory is that at the wavelengths concerned, radials get resolved as good enough to be a ground plane. So at he lengths recommended the radials recommended are probably appreciably smaller than the wavelength (~3m) so no issue. But how low do you want to go? Zero will probably go horribly wrong, as would 1 or 2. Three might be OK, but why not go for 4 or more unless there is some other limitation?

[radionortheast]

yup! like the guy says, ideally the groundplane should be just a soild peace of metal in a cone shape, of course that would be stupid and impratical, thought it would make a nice noise when the rain hits the side of it. you only need one radial to make gp work they have 3 which are evenly spaced to get an omidirectional pattern. from what i’ve been told when its in free space 3 are all thats needed that 4 do the samething.

you can make a gp aerial with just one radial, its about the simplest aerial you could use it won’t have an omidirectional pattern, bit more signal will go in the direction the radial is pointing in.

[NOYB]

Thumbs-up for "radionortheast". Can't see another way of doing that!

[Spokes]

Tuned one of the sirio ones up for my pal the other week, and it performed very well, they can hold more power than this design and they're very sturdy and neat, and they're not a great deal more than this one in cash either!

[halfwave]

Albert,
I forgot to say thanks for your great explanation and building
tips, (I ended up having to finish my lunch break at work early
as something came up I had to deal with), your knowledge is
much appreciated.

Can you remember what diameter of micro bore pipe you used,
As there are various ones available and if I try building one I don't
want to get something to small or to large.

When you connected the pipe to act as the main vertical radiating element
did you slide it over the central pin of the socket and pinch it around the pin
then solder it?

Thanks to everyone else for the comments on the amount of radials.

Spokes, I'm not going to be running big power to the antenna if I buy or build
one. But thanks for the heads up anyway.

[Albert H]

The first one I made was slightly over-engineered - I cut a short slot into the radiator so that I could fold and solder a taper that fitted snugly over the centre pin of the "N-type" socket.... overkill! The next ones had two loops of copper wire soldered inside the pipe, with the four short tails then soldered to the centre pin of the socket. I found that I needed a blowtorch (or a big soldering gun) to solder the wire loops inside the pipe, since the heat was conducted away by the bulk of the metal. This was an advantage, of course, when using a smaller iron to attach the wire tails to the centre pin of the socket!

The micro-bore pipe diameter isn't critical. I've used three different diameters, and the only differences were in weight and how easily bent they were! It's a compromise - use thicker pipe if you want greater rigidity.

Incidentally. it's good practice to close the ends of the pipes - I used to screw a nylon screw into them - and it's essential to paint it if it's going to get put up for any length of time. I've painted them green and brown when they've been put up trees, and pale grey when they've been on the top of a block. I would clean the copper with wire-wool then paint them with acrylic paint.

I found one of my old grey ⅝-wave ones (that was over 30 years old) up a well-known block in the Harrow Road about 18 months ago. It had been taken down and left on the domestic block roof - probably because it was too big to get through the roof doorway! The contractors I was working with retrieved it for me, and I got it home and tried it out. It still resonated at 90.4 MHz and showed a VSWR of 1.02 : 1. The bottom end of the coax was knackered where water had got in, but the "N" plug was wrapped in self-amalgamating tape and was OK. I cut about a metre off the end of the cable, and gave it a go.

In the same way, an old friend visited last week with a PLL rig I'd built in 1977 - 74-series TTL run at nearly 6.5V to get it to go at ~23MHz! It used a 23MHz VCO, buffer, doubler, filter, doubler, double-tuned filter, and two further "at frequency" stages just to get to 1 Watt! The dividers in the PLL were all 7490s programmed for the various division ratios that were needed, and it used a 4MHz crystal. The phase comparator was a dual flip-flop and NAND reset job, with the spare NAND gate used for lock detect and relay control. The PA was switched on about 5 seconds after the loop had locked.

The PA was a BLY87 into a BLY90, and gave about 55 Watts. The PA was on the biggest heatsink I've ever seen and didn't have a blower. The part of the rig that had failed was the PA supply relay. A new car headlight relay from Halfords, and it was as good as new! I spectrum-analysed it (I didn't own one when I built this beast) and was amazed at how clean it actually was! I'd taken a lot of care over separation of the various sections - especially to keep RF out of the logic. It still came up on the right frequency too.

[Analyser]

Albert, you really should have taken some pictures. The description of your old rig really doesn't do it justice, I'll bet