Make groundplane for a 1/4 wave whip

[wheresthetubes]

Hello,
I bought a 9ft 1/4 wave whip antenna, with the intention of testing out some 1980's handheld FM CBs, and a further mobile FM CB. I've subsequently found out I need a groundplane for the 1/4 wave to properly work.

The advice I've got is to make one; a 6" plate of alluminium, with a centre hole drilled out to accept a standard antenna, and with 4 alluminium poles that are each 1.5 metres long, and spread out equally from each corner of the 6" plate.

My questions;

where the whip antenna makes contact with the home-made groundplane, should there be a plastic washer applied so that there is no metal to metal contact between the antenna and the groundplane?

I seem to remember back in the early 80's when I dabbled with CB, I had to put a washer between a cb ariel and the biscuit tin (which acted as the groundplane) though this was very long ago.

Is it also essential that the radials of the groundplane should be 1.5m long, could they be shorter?

Many thanks.

[YT2095UK]

magmount bases work capacitively to the groundplane, but an electrical connection is fine too (gutter mounts are frequently electrically connected).
it shouldn`t make any difference as long as the g/ps large enough.
you can run 4 lengths of coax or wire from the corners too if you don`t have alu tubes.
you don`t want ANY antenna to GP connection though, it`s a great way to pop your output transistor.
test it with a multi-meter 1`st between center and braid of the coax, they should read no connection or resistance.

[Sean Williams]

Ground plane, or counterpoise, all the same thing.

300/F in megahertz will give you the length of a 1/4 wave resonant length.

However, for counterpoise uses, they can happliy be longer - the longer you make them the better performance you will get. Number of radials is also a good thing to maximise - simple insulated wire does the job well.

HTH
Sean

[wheresthetubes]

Thanks. I'd been advised that the use of coax as the radials would act as a good groundpalne for the whip. However I'm unsure how the four radials would connect to the whip. I'm assuming that the four equal sizes of coax (acting as the radials) would be connected to a 6" square alluminium plate. Where I'm a little confused is in the connection of the whip to the 6" plate.

Is there a metal to metal connection between the antenna and the plate, or is it important that there is no connection (with the use of a washer) between the plate and the antenna? An idiot guide here would be useful...... this is all fairly new to me

[Skywave]

Originally Posted by Sean Williams

300/F in megahertz will give you the length of a 1/4 wave resonant length.

Sean

I'm a bit puzzled by that.

I thought that wavelength (metres) x frequency (MHz) = 300. Therefore, "300/F(MHz)" will give one whole wavelength (in metres), not a quarter-wave.

Al. / Skywave.

[wheresthetubes]

Not as puzzled as I am Just done a little research on the web.... does the 6" plate have to make contact with the screen of the coax cable? Similarly, if no 6" alluminium plate was used, could I just connect the 4 radials/wires directly to the screen of the coax as it joins/connects to the antenna.?

[Skywave]

Originally Posted by wheresthetubes

Hello,
I bought a 9ft 1/4 wave whip antenna, with the intention of testing out some 1980's handheld FM CBs, and a further mobile FM CB. I've subsequently found out I need a groundplane for the 1/4 wave to properly work.

The advice I've got is to make one; a 6" plate of alluminium, with a centre hole drilled out to accept a standard antenna, and with 4 alluminium poles that are each 1.5 metres long, and spread out equally from each corner of the 6" plate.

Is it also essential that the radials of the groundplane should be 1.5m long, could they be shorter?

Many thanks.

You don't need to make a replacement aerial: what you have there is fine - it just needs a ground-plane, and the lengths of the radials that you have stated (1.5 m. long) sound about right; they could be a little longer without incurring problems. In theory, you could also shorten them, but I wouldn't. Note that this length (1.5 m.) is not one-quarter wavelength at 27 MHz, and contradictory to common belief, quarter-wave radials are best avoided when used with a ground-plane aerial. (See below*).

What you need to do is this.
You will probably find a co-axial connector at the base of your aerial. Arrange the metal plate so that it makes a good electrical contact to the outer of this connector. You do not want any electrical connection between the outer and the inner at the co-axial connector nor the plug connecting to it at the aerial. This is very important! Fixing the metal plate is often most easily acheived by drilling a hole (just the right size) in the centre of the plate, removing the four screws that are securing the co-axial connector to the base of the aerial, drill 4 holes in the metal plate so that the screws now go through the metal plate and into the co-axial connector - where they were originally. Your metal plate is now electrically & mechanically secured to the outer connection of the aerial. Now attach your four radials, one to each corner of the plate. You don't have to use the aluminium rods as specified: these are favoured simply because they are rigid and are self-supporting - therefore, very convenient. Yes, you can use wire instead - but not wire that is too thin - but do cut to the same length. However, if you do use wire, since it is not self-supporting, you will then need to fix insulators at the end of each radial and attach string or rope (note: non-metallic) and fix the ends of this string to four secure fixing points. You can see why aluminium radials are so favoured - a lot easier to erect!

The radials can run out horizontally or at an angle to the vertical aerial. Ideally, each radial should make the same angle with the vertical. The classic text-book theory says that you should adjust this angle for the lowest SWR at your transmitter, but something like 45 degrees is usually about right.

* References: "HF antennas for All Locations", L.A. Moxon, G6XN, p. 43, et al.; "Radio Communication Handbook", 6th. ed., RSGB, p.12.57 (The former reference provides a lot of relevant detail on this subject: too much to go into here!).

Hope that helps.

Al. / Skywave.

[Skywave]

Originally Posted by wheresthetubes

Not as puzzled as I am Just done a little research on the web.... does the 6" plate have to make contact with the screen of the coax cable? Similarly, if 6" alluminium plate was not used, could I just connect the 4 radials/wires directly to the screen of the coax as it joins/connects to the antenna.?

That Post crossed with mine immediately above: the answer to your first Q. is in that Post. But to perfectly clear, yes, the screen does need to be connected to the radials / metal plate - and NOT connected to the vertical part of the aerial; the inner of the co-ax connects to the vertical part only.

The answer to your 2nd. Q. is "no": the plate isn't absolutely necessary. But it does provide a mechanically-secure & easy-to-arrange method of connecting the radials.

Al.

[Sean Williams]

Hi Al,

Yes, forgot the divide by 4 bit.....

Call it a blonde moment....

Sean

[G8HQP Dave]

Note that this length (1.5 m.) is not one-quarter wavelength at 27 MHz, and contradictory to common belief, quarter-wave radials are best avoided when used with a ground-plane aerial.

So why do antenna textbooks (e.g. Krauss) specify quarter-wave radials? Could it be that there is some confusion between multiple (120?) ground wires augmenting real ground for a ground-mounted Marconi antenna (which do not have to be any particular length), and a small number of radials/counterpoises used for an elevated ground plane antenna (which should be near quarter-wave, in order to reduce coax outer current)?

I guess for low power CB it doesn't matter too much if the coax feeder radiates too.

I can understand why radials don't have to be exactly quarter-wave, as this mght give rise to perverse resonance effects, but I think they should be nearly quarter-wave. Of course, the 6" plate will have quite high capacitance so will replace some of the radial length. Maybe this is where the 'not quarter-wave' myth originates.

[GMB]

My own practical experience is that for small numbers of radials they do indeed have to be near 1/4 wave long to get a proper match, especially if angled downwards.

There is a lot of mythology about "earth". You need to keep in mind that it doesn't exist. All aerials consist of everything joined to the two terminals on the transmitter - and may include other leads that you didn't think were part of it - and your own body - and nearby objects.

The stuff written about Marconi-type aerials (i.e. a stick over ground) is an approximated simplification of this class of aerial systems. So never loose sight of the fact that these systems are modelled as an infinite flat metallic ground plane and the real world doesn't have one of those.

My other piece of practical experience is that the smallest balance error can result in standing waves that will confuse your socks off when tuning an aerial. I always add a pile of ferrites on coax - enough to be sure that I know where the aerial actually is.

[G8HQP Dave]

All aerials consist of everything joined to the two terminals on the transmitter - and may include other leads that you didn't think were part of it - and your own body - and nearby objects.

Hear, hear! If everyone who played with aerials had this pinned on their shack wall then a lot of myths and extravagant claims would fade into thin air.

[Phil G4SPZ]

Regarding ground plane antennas, these work by fooling the top 1/4 wave radiating element into believing that it is, in fact, one-half of a resonant half-wave dipole, the fundamental and simplest antenna. The ground, ground plane or ground-plane radial system creates a reflection of the top half of the antenna, and the whole thing presents an approximate feedpoint impedance of 50 ohms to the transmitter. It's usually the case that a closer match is obtained when radials are used if they slope downwards at about 45 degrees.

Going back to the original posting, though, I can't help thinking that walking around operating such a cumbersome antenna with a hand-held radio is likely to attract attention, and hopefully not from the Anti Terrorist Branch

I tried a similar arrangement many years ago, and found the weight of the antenna far too great to permit hand-held use of the radio. A better suggestion might be to mount your antenna on a vehicle, with a coaxial connection to the transceiver, and I think the results would be much better too.

[Skywave]

Originally Posted by G8HQP Dave

So why do antenna textbooks (e.g. Krauss) specify quarter-wave radials?
Maybe this is where the 'not quarter-wave' myth originates.

Yes, when I first read that, it came as a bit of a shock to me too!
But please read on . . .

As per the reference in my earlier Quote (The Radio Communication Handbook, 6th. ed., RSGB, pp. 12.57; 12.58) . . .
" . . . quarter-wavelength is the one length to be avoided since when close to resonance, phase is changing very rapidly and, allowing for finite length tolerances, the equalisation of radial currents becomes highly problematic."

In the other reference that I quoted from, L.A. Moxon goes on to say " . . .when full sized (i.e. quarter-wavelength) radials are used . . . these couple more tightly into the feeder system. Failure to adopt such measures (and here he refers to his previous paragraph where he discusses using radials less than a quarter-wavelength and ways and means of compensating for this) can lead to reduced efficiency, rf getting into mains wiring & causing TVI & various other 'nasties' ". He continues to report his experiments (at 14 MHz) using shortened radials . . . "and I obtained identical antenna currents and signal reports when comparing four quarter-wave radials with a set of four radials of 100cm. or two radials of 150 cm., using in each of the latter cases a common loading coil of 3uH".

Personally, I do not have the experience of experimenting with ground-plane aerials & quarter-wave radials and I have quoted from these two excellent references simply to assist the OP. However, I think it is fairly safe to say that these two quotes - and especially the latter one - come from sources that have had a great deal of experience in the design and use of aerials generally. Therefore, to write off their findings as "a myth" seems somewhat foolhardy.

Al. / Skywave

[G8HQP Dave]

If radials are more or less exactly quarter-wave, then two opposite ones can form a half-wave resonator. If there is any imbalance, due to slight differences in length or nearby objects, then at radial resonance you could get a high impedance 'ground' instead of the desired low impedance ground. So I can see that exact quarter-wave could be a problem. This would cause the TVI etc. which Moxon describes.

I am puzzled by Moxon's finding that good results can be obtained with very short radials. Does he put a choke balun on the feeder about a quarter-wave from the feedpoint? If so, he is using the start of the feeder as a counterpoise. If not, is he using the whole feeder as a counterpoise? I have found that even experts can believe or propagate myths, especially where antennas are concerned.

[Skywave]

Les Moxon states that he used a 3uH loading coil (as I said above). He also goes on to say that should any current be detected on the coax outer, winding the feeder around a ferrite core helps or changing the feeder length.

On using radials less than a quarter-wavelength, he is claiming from his own experience - and enough data is provided for his experimental conclusions to be verified by almost anyone. Perhaps later, I may investigate this for myself - but not at this time of the year!

However, perhaps we've drifted OT a little now - I do believe that the OP's Qs. have been answered; I'm sure he'll come back if this is not the case.

Al.

[Herald1360]

A useful coax(ish) connector for the bottom of the whip is a PL259. It's cheap and fairly robust. I used to have the mating socket directly mounted to the roof of a 400E Thames in the mid 70's, with a welding wire whip for 2m. Not as good as the popular 5/8 setup, but seriously cheap. Worked OK on the AM car radio, too.....

Chris

[wheresthetubes]

Thanks Skywave for the instructions, they seem easy to follow. I will be having a go at fixing a groundplane to the whip in the next couple of days. Adjusting the angle of the radials for the lowest SWR... I would never have thought of that. The screen of the coax will need to make contact with the 4 radials via the metal plate then, so I guess I'll have to solder the screen down to the plate? I'm hoping this is not the case as I plan to make the radials as a kit... the whip itself comes in two parts.

I had read on a couple of websites that the radials would need to be the same length as the whip (about 9 ft) so as to creat a pefect half-wave, but the message I'm getting here is that this is not the case, or maybe not the case if I used more than 4 radials?

If I get stuck somewhere along the way whilst making this I will post a couple of pics, and hopefully someone will point me back in the right direction. Many thanks for all who have contributed to this thread.

[Skywave]

Originally Posted by wheresthetubes

I had read on a couple of websites that the radials would need to be the same length as the whip (about 9 ft) so as to creat a pefect half-wave, but the message I'm getting here is that this is not the case, or maybe not the case if I used more than 4 radials?

Hi! As I said, based on my understanding and what I have read in sources by others who I credit with having the necessary expertise, comprehension and experience that I lack, the basic raw materials that you have in your OP (Opening Post) should be just fine - to get started with anyway. All aerial design work involves an element of cut-'n'-try; the theory and the mathematics behind it is there simply to narrow down the number of possibilities - and thus speed-up the required result.

So, in summary: fabricate the ali plate: holes & fixings & the RF connector; attach the radials you have to the four corners of the plate (flatten the ends of the radials and secure them to the plate with nuts & bolts); fix the aerial in a place so that is well clear of earthed objects and metal structures and (say) at least 2 metres above the ground (although this is not critical). Connect up to the TX and assess the SWR. Adjust the angle of the radials for optimum SWR - which will change somewhat as you change frequency. Then when all seems OK, finally tighten the radial securing screws and apply paint: this helps stop corrosion which will upset the radial-to-metal plate conductivity if left untreated.

Good luck!

Al. / Skywave.