CB aerials

[crestavega]

I'm blowing the dust off the old box of CB radio bits with a view to getting the kids interested in "real" radio comms whilst we aren't allowed to visit or have friends over etc.

In considering what antennas to use, I realised that some of the most popular old CB antennas appear to be "top loaded" that is to say, they have a loading coil (?) at the TOP of the vertical antenna. Chiefly I am referring to the ubiquitous DV27 (made by Sirio I think) and the famous "Firestik". looking carefully thru the heatshrink, the antena wire is visible on these antennas. the Firestik appears to be a rubber-duckie on acid - it is helically wound throughout the 8' blank but the spiral gets tighter and tighter as you go up the antenna blank, ending up as a tight coil at the top. The DV27 is similar, but seems to be a straight vertical wire along the blank terminating in a tight coil at the top of the antenna (just before the adjustable SWR tip)

Certainly many CB antennas have a loading coil at the base, which seems the obvious place to put it, and this I can (just about) understand. Some have a coil in the middle, which I always thought was purely an aesthetic gimmick (the "spiked beercan" antenna), but perhaps not?

I understand that electrically, you can put a "lengthening" coil anywhere in series with the antenna, in order to physically shorten the antenna for any given wavelegth, and where you put it will have implications on the inductance required (and the feedpoint Z I expect). But I wonder WHY in some popular antennas the coil is at the top (or indeed in the middle). is there some specific performance advantage?

Cant find much about it in my cursory googlings for homebrew verticals. All the designs advocate putting the coil at the bottom of the vertical, which certainly seems like common sense. but I would like to know WHY the coil on mobile CB aerials was not always at the base.

Thanks
T

[Herald1360]

The bottom isn't the best place, except mechanically. Apparently about 2/3 of the way up is about best. ISTR it's got something to do with current distribution and its effect on the radiation efficiency.

https://forums.qrz.com/index.php?thr...effect.446603/

discusses it.

Isn't there some legal constraint on how big the antenna can be that's the main issue here? Otherwise for a fixed base station you could use a full size quarter wave vertical- it would only be about 8.5 feet long. Or is the loading coil setup just for mobile rigs?


David?

[crestavega]

Thanks for that.
funny, I had a vague idea that it was something about better current distribution on the radiator element too.

I guess Ill just make one and see what the hell happens.
there used to be some regulation about CB antennas gain and ERP which certainly had an effect on their marketing and was responsible for the -10dB switch on 27/81 rigs but I understand anecdotally the reg is no longer in force.

the vast majority of CB antennas floating around junk collections are mobile cos it was chiefly a mobile fad, hence my quesstion.

[Jon_G4MDC]

1.5 metres maximum radiator length was the stipulation.
But you could have any length of counterpoise on the earth side and so the fashion developed for mounting wot-poles upside down, presumably with the feedpoint connections reversed....

[electronicskip]

The ruling when CB27/81 was legalised stipulated that all mobile twigs(aerials) should be 1.5 length and base loaded.

All now relaxed including useage of AM/USB/LSB on approved rigs of which i have a President Jackson 2 classic.

[Aub]

All now relaxed including useage of AM/USB/LSB on approved rigs of which i have a President Jackson 2 classic.

That's interesting. Presumably, the authorities are not bothered about TVI, since the change to digital TV, or was there another reason for the relaxation?

Cheers

Aub

[SiriusHardware]

General indifference to what happens on HF, I think, Aub.

As 'Skip intimated, AM / SSB are still only allowed on what people would call 'Mid' band, and only with equipment specifically type approved for use after these changes came about, which of course excludes every item of classic equipment you might be able to bring to mind.

I agree, use of AM/SSB modes does increase the likelihood of interference because both modes are more easily decoded back into something audible by simple transistor junctions in amplifiers, etc.

[Electronpusher0]

In the height of the CB craze a friend of mine made a "Slim Jim" Antenna for CB using wire for the elements and supended from the top of a tree by rope. It was massive.
Would not be legal but back then neither was CB.

Peter

[crackle]

I can vouch for the DV27 still being the best of the bunch for the size.
Mike

[SiriusHardware]

For build quality I used to favour 'Valor' mobile aerials, the most well known models of which were the 'Half Breed' (Base loaded) and 'Warrior' (Centre loaded). People were obviously slightly less obsessed with the notion of 'cultural appropriation' in those days.

Valor also made the original 'Firestiks' although I personally couldn't get on with them as they were rather heavy for their length and quite likely to buckle the bodywork if you had them mounted on the wing of a British car rather than the wing mirror of a PeterBilt or Mack. They were also very fiddly to SWR, you had to pull the top cap off, unwind a bit of the wire and snip bits off until you hit the lowest SWR. Later versions concealed an adjustable rod tip under the cap, similar in principle to the ones on DV27s.

Valor also produced the original versions of the 'Pro-Am' single band Amateur HF mobile aerials, later copied as 'Am-Pro' and other brands: But the Valor versions were the originals.

The K40 also deserves an honourable mention, although base loaded it was a very well made aerial with a built-in quick-release arrangement on the mount.

[electronicskip]

I use a K40 aerial with the matching mag mount when im using my CB mobile.
Had the same aerial since about 1982 and its still made today although its rather expensive for what it is.

[crackle]

I have an original Half Breed, I can vouch for the build quality with its nice tapered stainless steel whip, and a centre loaded Modulator II. I recently did a comparison with the DV27 and on a contact between Basildon and Rochester the DV27 gave better results.
But overall my 7 foot, 5/8th Firestick would win hands down, but I cant drive around with that on the roof.
Mike

[SiriusHardware]

The nearest contemporary CB equivalent of the amateur Pro-AM aerials - half helical and half whip - was the 'Bullwhip' which was quite a long aerial. I used to drive around with one of those on the roof of a Fiat 126. I had it on a DV27 style mount with the slip joint aligned along the front to back axis of the roof, so if it hit something it would just be swept backwards and not do too much damage.

Tandy had some nice mobile CB aerials as well, I have a very well made centre loaded Tandy aerial, the loading coil being a solid light grey colour. It was originally sold with a (truck) mirror mount, but I haven't seen the mount for quite some time - I bought it originally to use it as a loft aerial and I mounted it on a square plate with long, thick sloping wire radials reaching away into the corners of the loft. That was in the days when Tandy were doing a roaring trade in CB accessories despite the fact that they still couldn't sell CB radios.

Of the aerials available today, I find that the long version of the 'Stinger' type, basically just a whip with an open shock spring / loading coil, often seen on 4*4 vehicles, has the widest low SWR range of any of the types I have. Mine has a useable low SWR, below 1.5, through both of the UK bands without having to optimise it for one band or the other or use an ATU.

[G6Tanuki]

The Valor "Half Breed" - yes I remember them - they were popular CB antennas in the early-80s and I must have converted - for the price of a couple of pints - at least 50 0f the things to '5/8-wave-on-2-Metres' antennas for the ex-CBers who got their B-licence.

[Hint: a 5/8-wave on 2M also works well as a base-loaded shortened-quatere-wave on 50MHz!]

Pragmatically, on 27/28MHz the further-up you place the loading-coil the greater the radiation-efficiency. The highest RF current - which provides the radiation - is greatest at the base and tapers-off the further you go up the antenna. Moving the loading-coil [which is best imagined as "a bit of the antenna coiled-up but which can;t really radiate that much" - further up the antenna means that more of the high-current - and hence high-radiation - part of the antenna [the part below the coil] can do its thing.

Downside is that the further-up the antenna you move the coil the higher the 'Q' becomes and so the narrower the bandwidth over which you can get a good SWR.

The "Firestik" type antennas work well - because even though they use 'distributed' loading, the lower part is really not that much of a coil - it's more a wire coiled round the core a few times for convenience so can be treated largely as a straight wire. There was also a longer version of the DV27 called the DX27 - these were about six feet long, with a thicker fibreglass 'rod' and lazily-coiled winding round the bottom part of this as in the Firestik.

[ionburn]

Originally Posted by G6Tanuki

There was also a longer version of the DV27 called the DX27 - these were about six feet long, with a thicker fibreglass 'rod' and lazily-coiled winding round the bottom part of this as in the Firestik.

Although I don't have CB radios any more I still do have my old DX27 setup in the loft, from the last time I had interest in those frequencies. I cannot say that I did any tests with it but it did work reasonably. I never thought it particularly long.

[Jon_G4MDC]

I was very shocked when I found out that lately Stingers seem to be made of
painted steel and they go rusty. Was it always so?

Then there was the one which was tuned by an aluminium ring which screwed up and down. The shorted turn?

DV27s did work well but it was easy to set them on fire, up top, by running QRO.
Firestiks less so - which was sort of a fun revelation.

[G6Tanuki]

The one with the adjustable aluminium rings was the Valor "Dial-a-Match".

It was indeed a shorted turn which 'slugged' the base-loading coil.

Actually it was two separate shorted-turns rings which you were meant to screw-together when the 'match' was found.

Alas they often corroded together by the action of road-salt-spray etc and could thereafter never be moved.

[SiriusHardware]

Ah yes, Valor were doing so well... and then they came out with the "Dial-A-Match". Not their finest hour.

Jon, I bought my Stinger quite a few years ago when they were first being sold by Maplin, so I can only assume mine is the real thing as it isn't showing any signs of rusting on the one or two places where the paint has been knocked off, and it has been on many a week long journey around Scotland where it got, obviously, wet.

Like everything else, Stingers will have been cheaply copied if there was money in it. With something with such a simple form factor, who can tell just by looking whether they are fake or not?

[Dave757]

Hi,

A friend of mine used a Dial A Match on his garage roof as a base antenna.
He was unlucky enough to suffer a direct lightning strike which melted the antenna
and left a burn line all along the route of the coax including across the table to the
rig - a Tristar 747 if my memory serves me right.
Unsurprisingly the radio no longer worked, and he asked me to have a look at it.
What was surprising was that the radio was ok except for the idiot diode and the
resulting blown fuse.
He was very fond of that set, and not a bit concerned about the burnt wall,
paintwork, carpet and table, unlike his wife, whose views were exactly the opposite!

Kind regards
Dave

[Joe_Lorenz]

Hello,

my real favourite was the one quarter lambda DV 27 "long" from Sirio, 2,75 m long. Still today I have got a "Tank Whip" that is same size but all stainless steel, with a big coil spring at the base.

These were street legal in Germany if over all vehicle height did not exceed 4 m. So either buy a Lambo ... no choice if just enough money for an Opel ... or drill a hole in the rear chrome bumper and mount it there.

Working very well but giving a strong directional effect.

I should start digging my basement to locate that equipment once again!

Regards, Joe