Hi-Gain Vertical Half Wave Antenna

[Barry06]

Has anyone successfully made a working replica of this antenna?

I’d like to buy one if anyone can build one to sell.

[Gigahertz]

Really surprised with the details available someone is selling them on ebay by now.

[RF-Head]

link ?

[mixfm]

used one back in the day [ better then a dipole ] our tech guy built it the man, offcock have it now lol

[Gigahertz]

link ?

Sorry I meant to say considering the details are available on here someone hasn't got them made up and flogging them by now.

Teach me to read what I write

[Gigahertz]

More for giving it a try and as this thread had seemed to go quite. I can assume no one has built one from the details provided by Lee???

I started the 50watt version out of bits I have from an old CB antenna, some metals enclosure that I cut for the plates. Just waiting on some nylon bolts and will put it together.

Just wondering if anyone can point me in the right direction. I can't see anything mentioned in the documentation about the coil winding on the cold/earth side for the 50watt version (unless I missed it). Pictures shows 7 turns of enameled wire on the higher power versions but I think I can see 3-4 turns of brown insulated wire on the low power version? Just read bottom coil is 7turns on all versions

Also where there any details of the distance between plates. As this forms a capacitor was any values given or is this trial and error.

[Gigahertz]

Just a start on it.

[Frequent Lee]

The size of the plates is critical and as you rightly assumed the air space between them is to tune it, start of with them being approximately 1cm apart and that won't be far off.
The 50 watt coil is just solid copper core from twin and earth mains power cable hence why some were blue sheath and some brown.

The bandwidth is about 5mhz either side of the tuned length of the radiator.

Claimed gain is 4.6dbi with a narrow angle of radiation, get them high up and mounted in free space and they hands down out perform a twin stack or a J pole.

I used to get them for £25 each built as I had known Stephen for over 20 years and put a lot of business his way.

[shuffy]

Just a start on it.

I already love it.

[Albert H]

The big problem with a ½-wave aerial is that the feedpoint impedance is (theoretically) infinite. In reality - because of materials losses - the impedance is just very high (Stephen estimated it at 5 - 6 kΩ). The big trick is getting an efficient impedance transformation, so that the antenna matches 50Ω.

Practical experience showed that the SWR was virtually flat over about 2½ MHz either side of the designed frequency, and rapidly rose to about 2 : 1 by 4 MHz away. The gain - at frequency - is around 2.8 dBd (gain over a dipole) and is truly omni-directional. We measured the real gain on an antenna range. It needs to be mounted in free space at least a wavelength (~3m) above anything else. The radiation pattern is flattened, with very little energy going upwards - it radiates in the direction you really want!

Practical experience showed that it was a very effective (and cheap to construct) aerial, and would withstand the rigours of the worst of European weather. I never liked the inverted plastic funnel over the capacitor, and made up some proper covers that had (effectively) a funnel-shaped top, with a tin-can sized lower section that screwed into threads in the upper section. The entire cover was made out of a kind of PVC plastic material that could be (quite coarsely) threaded, and provided complete water-proofing for the matching system.

Friends of mine experimented with various means of matching this aerial - mostly in an effort to eliminate the home-made capacitor - and also tried to construct a "general" version of the aerial that could be quickly tuned to anywhere in Band II with a variable length radiator. They had some success, but found that Stephen's version was probably about as good as possible.

The "Flowerpot" aerial is a relative of this design, and can also be cheaply made, and has the advantage of easier matching, but has less gain. One interesting version of the Flowerpot is the 0.64-wavelength type. This can have gain comparable to Stephen's half-wave job, but has narrower bandwidth and is more critical to tune.

[Gigahertz]

Just a small updated!

Fitted the bottom coil, decided on 3 - 1/2 turns as the of the tube is larger and if that doesn't work will change it to the 7 turns (coil).

Also fitted the plates. Nothing has been set up yet just wanted to get it built and will add coax and start checking with analyzer and see where I'm at.

Everything could do with tidying up but this is only testing stages.

[Gigahertz]

The size of the plates is critical and as you rightly assumed the air space between them is to tune it, start of with them being approximately 1cm apart and that won't be far off.
The 50 watt coil is just solid copper core from twin and earth mains power cable hence why some were blue sheath and some brown.

The bandwidth is about 5mhz either side of the tuned length of the radiator.

Claimed gain is 4.6dbi with a narrow angle of radiation, get them high up and mounted in free space and they hands down out perform a twin stack or a J pole.

I used to get them for £25 each built as I had known Stephen for over 20 years and put a lot of business his way.

Cheers Lee for info!

You were getting them cheap back then

[Gigahertz]

Another update.

Well quickly added a short lenght of RG-58U (apprx 5M) and went outside to the garden.

Measurements I followed were for 87.9MHz

Just holding the antenna in my hand in free space 6ft off the ground and Lee's guidance of 1cm plates apart I had a good match on 90.5Mhz

Yes the antenna and analyzer are laying on the ground in the photo but this is after I had taken a sweep with the analyzer. Results are promising so will mount on some poles and do some more checks and tweaks hopefully on Sunday.

[fmuser877]

wonder how far it will go?

[Gigahertz]

wonder how far it will go?

What like a javelin when I throw it ?

I will do some tests and compare it with a Sirio 1/4wave mounted at the same height

[Gigahertz]

Well mounted it on a 6ft pole in the garden has 5m of RG-58 connected.

Here's the results.......... As mentioned I was aiming for 87.9MHz. So either the coils need to be wound better/slighlty longer or adjust the length of the whip.

But happy with it so far with a the amount of time spend on it. Once things are finialised doubt it would take anybody more than 1hr to make when you have all the bits.

[shuffy]

As part of a reboot of an old project, I've had another crack at a version of the NRG half wave vertical. It's not quite finished but I thought I'd share some pics, results and other observations.

I struggled with my previous attempt, which had the whip spaced away from the counterpoise with bits of perspex, a split matching coil like the original antenna, and a beehive capacitor completing the matching network. I followed the theory, but there must have been something "off" in the construction of it - tuning was very difficult, I was never happy with the SWR and performance wasn't good once I managed to get an SWR good enough to connect a real transmitter to.

My current version looks more like the original or Gigahertz's. When I did the version with the beehive, I hadn't managed to get my questions around Stephen's plate capacitor answered, and I was unsure how he'd attached the whip to the mast and the matching network. Eventually I followed my instinct around the capacitor (had to be tin plate for soldering?) and I came up with a 3D printed "insert" (now known, thanks to my former technical director, as the "butt plug") which sits inside the counterpoise and supports the whip. Headlines are:

• 2-part half wave whip made from 8mm aluminium tube with 5mm inner insert.
  Whip mounted inside 3D printed spacer in 1" aluminium mast. Spacer also serves to mount the top capacitor plate and matching network inductor. An early version snapped so this requires parts of the "butt plug" to be strengthened
  Capacitor plates made from tin plate cut to template using tin snips. Plates spaced using M4 nylon nuts and bolts.
  Prototype uses single inductor in matching circuit to easily move the tap and find the best feed point. I don't see an issue in changing it to 2 coils as per NRG original.
  Counterpoise is very short (50cm?), part of an old antenna. Will be "bulked up" by attaching to mast with copper pipe.

I tested a lash-up version with a nano VNA which I normally use without issue for tuning other antennas (J poles etc). Couldn't get a sensible reading with this antenna though, and I'm still not sure why. I did however persevere using a 5W TX and SWR meter, with encouraging results even inside the house in proximity to walls etc (SWR about 1.3:1) but outside in free space, less than 1.1 with very good bandwidth around resonance and rudimentary tests of the radiation pattern were encouraging.

It's a bit scruffy at the moment but if I get the chance I'll tidy this up as I now think it's worth persevering with.

[FMEnjoyer]

1/2 wave end fed will be no better than a 1/4 ground plane antenna. It will have no gain over a dipole either as it is an end fed dipole.
You might need to choke the feed line otherwise any slight radiation pattern improvement over the 1/4 wave will be lost through feed line radiation. They are voltage fed so usually common mode on feed line should not be too bad.

The only possible advantage is mounting and no radials, it will also lose a little RF energy as heat in the matching coil meaning losses over a dipole.

Hope it works well for you.

[radionortheast]

I think the coil was more an inch wide wound round the bottom, don't know if that helps, fairly thick wire too, some kind of plate acting as capacitor, I had one but took it apart.

[FMEnjoyer]

If there is anything metallic in the reactive near field of an antenna then all PC plots are out the window. A 1/2 wave dipole has a very slightly lower take of angle than a 1/4 wave. Needing a coil = losses at all angles, not much but it is there. The current centre of all the antennas are in the middle a 1/4 is really half of a dipole with the ground plane x 3 or 4 put 8 or 16 radials for even better efficiency actually that might beat a dipole. The GPs is in essence just the other pole you just have 4 of them and if at 45 degrees you get a very good impedance match with no coil required.Half wave end feds can be great, they can be convenient for some mounts and intalls, cb antennas are just that.
Hams will say you need 0.05 lambda counter poise. CP is an odd thing to me, as I never quite understand why the cmc would take the path of the counter poise to flow rather than the coax shield. The more current is on the CP or coax shield the more radiation from those sources that will mess your radiation pattern up. The longer your CP is then further away from a 1/2 wavelength antenna the antenna is and will radiate differently and not favourably, it will be worse. So choke the coax is sensible, and keep cp short a bit more coiling a bit more loss. It is really difficult to improve on a dipole and a 1/4 wave ground plane.These gain figures for a 1/2 wave antenna that is not a dipole of any kind are fiction. Even with a 5/8 wave it is very difficult to actually get that supposed gain. You also have to take into account what is under the antennas including a conductive mast, often unknowable in every situation that will also affect the pattern. Use whatever is easiest that is the best antenna for any situation.Actual results in the real world tx site are unknowable. If you are doing it for experiments and enjoyment thats good but the differences will be very small despite anecdotal stories 'once I put an antenna on a big thing somewhere and it went really far' right everyone did that.