I recently built a ground independent UHF CB ANTENNA and was so chuffed with the results I thought I would post my efforts here . . .

The technical details are fully covered on another platform (Gold Coast Amateur Radio - on Facebook - admin please delete if this link is not allowed) but suffice to say the design is based on an iteration of the ZEPP ANTENNA or by a common name "J-Pole" after its resemblance to the letter "J" although this one is a double J (if any of this makes sense) - the ZEPP, is so named after the Zeppelin Airships of by gone days - the hardware is included along with where to find the bits and pieces.

I hope you have as much fun as I did with this awesome little antenna.

You will need a 25mm round electrical conduit "joiner" about 25-30mm diameter from Bunnings that will form the base assembly.

Two end caps that fit inside the joiner from Bunnings, cost will be under $5.

A M10 Stainless Bolt about 30-40 mm long and a Nylock, cost at Bunnings is about $5 for a packet.

One 2.4mm stainless steel welding filler rod from any welding supplier, cost will be $2.

A 3m or so length of RG58 50 ohm coax long enough to reach and suit your job from Jaycar, cost around $5.

A PL259 coax plug from Jaycar, cost about $3.

Start by drilling a 2.4 mm hole under the head of the M10 bolt and all the way through the shaft of the bolt (photos to follow the next day or so). Feed the stainless steel welding filler rod through the hole until you have about 160mm sticking out.

Then carefully measure 158 mm - bend this length at 90 degrees along with the other longer end at 90 degrees to form a tight U shape.

Measure the longer end and mark 158 mm along the longer length/side and another at 316 mm (474 mm all up).

The welding filler rod comes in lengths around 900mm so there is ample to complete the job.

Then drill a hole in one end cap for the M10 bolt and push the bolt through - use a spare nut to tighten the bolt on the outside or base side so that the short length and long length of rod are clamped tight pointing sky ward.

Bend the 474mm mark around a broom stick or similar and fold the other side 316mm long half wave length back towards the short stub and trim off the excess leaving a 2-4 mm gap.

Accuracy is important but any tiny errors will be tuned out when you adjust the VSWR - for now it is enough to know that your antenna is as close to a resonant length as possible.

It should resemble an elongated oval shape about 480mm or so long.

The stub and the folded end should not touch. There should be a 2-3mm gap.

Then prepare your coax feed it through a hole in the side of the joiner as per below, and fit off the PL259 and bare the braid and centre conductor on the other end and tidy up the tag ends by twisting and soldering/tinning the tags.

Simply, the two 158mm stub and the folded end form a folded dipole, and two 316mm folded half wave lengths are the active or radiator part of the antenna - the stub or stubs are parallel and carry equal and opposite current and voltage so cancel each other out leaving the two folded half wave dipole lengths to radiate - every half wave length the alternating current of the radio signal reverses, and the magic so to speak comes when two half wave length elements are folded back on each other so the current is "in phase" doubling the gain of the antenna. Or so the text books tell us.

Don't worry if my babble doesn't make sense - just trust me when I say the theoretic gain is double that of a simple dipole at 3dB.

That's 3dB over a dipole not the commonly quoted isotropic radiator which is -2.14dB compared to a dipole (manufacturers like to reference their antennas against an isotropic radiator as it makes out the antenna is more powerful than it really is). So in reality this is a 5.14 gain over an isotropic lol.

Now measure up about 40mm up from the bottom of the stub and the head of the bolt and twist the tag ends of the coax centre conductor and braid around either sides of the stainless stubs.

Yes - IT IS NOT A FIXED JOIN AS STAINLESS AND LEAD DON"T EASILY HOMOGENIZE.

The centre conductor goes on one side of the stub and the braid to the other stub equal distance and opposite if this makes sense - solder as best you can to get a good electrical contact. Its all a/c and is forgiving to a point and will perform fine.

Drill a small hole about 45 mill from one end of the conduit joiner and feed the coax through (before you put the PL259 on) as mentioned above.

As the folded ZEPP is ground independent it does not need to be earthed to the body of your RV and will work just as well if mounted on a fiberglass body RV or if you hold it in your hand lol - but be careful a radiation called radio frequency radiation can and will burn - AND unlike a heat burn will take ages to heal.  So take care OK.

The M10 bolt thread should stick out the bottom of the end cap and be a VERY strong mount to fix to your favourite antenna mount bracket.

Then you will need to fine tune the antenna.

Here's the tricky part - as simply as I can state it, somewhere between the bolt at the bottom of the stub which is "closed circuit" and the top of the stub which is "open circuit" there is 50 ohms that perfectly matches your transmitter and coax feed line impedence.

To find the exact position you need to use a VSWR meter - don't be afraid of getting one and trying this out for yourself).

Some of you will remember these as "swer" meters from 27mHz days.

These are not expensive to buy second hand and are good to keep handy in the future - be sure it is a model that works on UHF - check around if you don't have one and you should be able to get a good second hand version for around $50.

Anyway somewhere around 40mm from the bottom of the stub is a good starting point when checking the VSWR - my first attempt was 2:1 which is 100 ohms so I carefully tapped both of the centre conductor and braid down about 1mm - and repeated this until the VSWR dropped to 1:1.1 or 55ohms which was a low as I could get without being OCD about it and spending hours.

If the VSWR climbs up when adjusting the centre and braid try sliding it the opposite direction. This process can drive you around the bend or be a breeze - I have experienced both so be patient and your efforts will be rewarded.

Keep test transmissions as short as possible while tuning to avoid damage to the final transistors - a badly out of tune antenna (I have seen heaps of them) can blow the finals and end in tears - I recommend that 2:1 and greater as being dangerous . . . .

Once the VSWR is at or very near as possible to 1:1 your antenna is considered tuned - mark the locations with a black making pen in case you are as clumsy as me so you can retune it as you finalise the build.

Carefully fit the joiner over the folded dipole end and glue it to the end cap with the bolt - forming a sealed cylindrical sealed base assembly - again carefully feed the other cap over the folded dipoles and just before you seal it check the VSWR as ZEPPS are notoriously famous for reacting with nearby objects.

Mine needed a very tiny adjustment that was probably due to careless handling.

When you are happy seal it with a dab or ten of glue.

The coax should stick out through the conduit joiner base assembly at right angles to the antenna and will need a drop of glue to seal out moisture.

I used some heat shrink to keep the stub end and the end of the folded dipole from touching and or moving about in the wind.

The stainless is utterly bullet proof and resilient to everything except for low branches or roll over - so should last many years.

Your antenna is now ready to mount.

I always try to keep the vertical folded dipole radiator away from any other vertical metal structure etc to avoid capacitive coupling and de tuning your antenna - regardless of whether it is a home brew or store bought IT WILL DE TUNE when near other objects - even another antenna - remember to keep your distance and social distance the antenna lmao.

I found the compact size and excellent performance very satisfying - it absolutely dominates the channel with clear crisp transmission and reception. I hope to do a 6dB double double soon with a full suite of pics - but for now am pleased to run this and post with a few pics of the finished item below.

I hope you are able to follow my suggestions and can duplicate the build and are not put off with the VSWR meter and tuning etc.

Let me know if you have any questions . . .

Or say hi on Channel 40 if you see my little Hiace Van around

7.3. from Aaron formally VK4VOX (0416 480 371)

-- Edited by Aaron on Thursday 2nd of September 2021 12:29:01 PM

-- Edited by Aaron on Thursday 2nd of September 2021 01:02:23 PM

I am having trouble uploading images lol - give me a day to sort it. . .

Aaron: your text appears as a solid block with no paragraphs and is, essentially, unreadable.

Whilst I'm posting I'll take the opportunity to ask if you have modelled your antenna and, if so, you may wish to post the plots?

hi Mike - thanks for the heads up - I couldn't sleep last night (old timers disease lol) so did it all on my phone n the dark - let me have a crack at the formatting - let me know if you have trouble - Aaron 0416 480 371

Aaron wrote:

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hi Mike - thanks for the heads up - I couldn't sleep last night (old timers disease lol) so did it all on my phone n the dark - let me have a crack at the formatting - let me know if you have trouble - Aaron 0416 480 371

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 Hi Aaron.Not questioning Mike's assessment of your post,but on my iPad it is perfect,and easily read? Full stops,commas,capital letters etc,and the sentences are sensible.Paragraphs may help,but on iPads etc I don't usually use them either.Anyway,hope this helps? Cheers

AWESOME yobarr - thanks heaps for the bouquet - so much better then brick bats - hope you build one and get as much pleasure out of the result as me.

Aaron wrote:

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AWESOME yobarr - thanks heaps for the bouquet - so much better then brick bats - hope you build one and get as much pleasure out of the result as me.

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 Thanks Aaron,but you can be sure that I won't be building one! I know about vehicle capacities,and weights,and stuff like that,but have little to no understanding of anything technical.Not to say I couldn't learn,but to be completely honest,I simply am not interested.The UHF,the GME PLB and the Sat phone will do me,as all I have to do is send a text and the person with whom I wish to converse simply calls me back at standard mobile rates. But thanks for the thought! Cheers

The formatting is now good Aaron.

Have you modelled you antenna?

STANDING WAVE RATIO - abbreviated to SWR and as the Guru once stated "more correctly VOLTAGE STANDING WAVE RATIO (VSWR) is a simplified math function expressed as one impedence divided by another - for example 100 ohms divided by 50 ohms = 2.

In other words if your antenna has a Vswr of 2 the impedence of the antenna is 100hms.

Another example (and as contentious as it is around some circles) a "resonant" centre fed dipole has an impedence of 72.5 ohms - lets call it 75 to make the explanation easier - therefore a dipole will display a Vswr of 75 ohms (at the centre of the dipole) divided by 50 ohms (being the impedence of the coax feed line) which results in a Vswr of 1.5.1. WHAT ! I hear some of you shouting in disbelief ! All true. IF you have measured your dipole correctly the Vswr should be 1.5:1. So don't go trimming it until it hits 1:1 or you will be operating a non resonant dipole and will experience loss of performance.

So to a quarter wave ground plane antenna will have a impedence of half a half wave dipole of 37.5 ohms or a Vswr of 50 ohms (coax impedence) divided by 37.5 (antenna impedence) which equals 1.33:1.

Sorry for getting long winded - but there a a lot of myths surrounding antennas going back to the good old 27 mHz days.

My neighbour happily operates his store bought antenna on his Tojo Cruiser bull bar that measures a Vswr or 2:1 or 100 ohms - this means about half of all the 5 watts of power coming from his UHF CB transmitter is lost in the mismatch between the radio, the feed line and the antenna when he transmits, AND about half of all the incoming signal is lost on reception due to the same mismatch. Akin to talking while gagged and listening with ear plugs.

Of much more serious a matter is what is happening to half the lost power during transmission.

Because half of all the power is not being transmitted it is in fact being reflected and being measured as a standing wave ratio of power out and power reflected. AND here's where sh-it gets serious. IF a standing wave peaks at just the right distance along the feed line (coax) and IF that peak occurs at the set AND in particularly the final power amplifier transmitters then that power can permanently damage those transistors rendering the radio useless as a transmitter. Simply put the power has to go somewhere and if it goes somewhere like the finals then you can be left with a radio that doesn't work just when you need it most.

All the IF's and AND's aside I always try to carry a UHF Vswr meter so I can check the antenna should the need arise.

To recap Voltage Standing Wave Ratio is a function of one impedence divided by another impedence and can be used to calculate the impedence of an antenna for your peace of mind or for service or replacement of the antenna to keep your vitally important radio in tip top shape.

Enough for now.

Thanks again.

7.3. Aaron formerly VK4VOX

I always try to travel with a Vswr meter as it can be easily used to check on the serviceability of an antenna in the event of a tree or animal strike or god forbid a rollover.

IF I have damaged an antenna and it IS an emergency THEN I know I can tune a piece of fencing wire to call for help.

But all the IF's, IS's and THEN's aside, to each their own.

Thanks for the excellent feedback.

7.3 from Aaron formerly VK4VOX

Hi.

By modelling do you mean have I built and tested one for UHF CB???

if the answer is "yes", then yes I have for the first time on UHF CB - and other frequencies as this design lends itself to marine VHF and Amateur Radio EXTREMELY well.

If "no" then please forgive my lightening fast mind and explain what you mean by modelling ???

Thanks heaps for the feed back.

Aaron

Aaron wrote:

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By modelling do you mean have I built and tested one for UHF CB???

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Hi Aaron

"Modelling" is a mathematical process performed by a computer, it takes all the known variables of a problem as an input and computes an output solution. It is, nowadays, widely used in everything from medical issues to space flight.

There are a number of free modelling programmes for antennas the best know perhaps is 4nec2 which generates SWR, lobe plots and all sorts of other good information about your proposed antenna.

Another excellent and free, electronics, modelling programme is LT Spice which just keeps getting better and better as the years pass.

Such software requires a basic understanding of the issues one is trying to model and also a learning curve for the software itself but the results will repay your efforts many, many times over and the potential for improving one's knowledge from experimenting is enormous. 

Hi Guru (not the person of questionable ethics that I once knew I hope !)

I have never used or even heard of a antenna modelling software package before - so am intrigued - thanks for the suggestion.

As I am old school and in over 40 years have relied upon accuracy of measurement and careful build techniques to safely see me through a build. I have a 30 year extended double built from structural grade marine aluminium that is still in perfect working condition having withstood many a storm and a number of cyclones. I have also built hundreds of these for hobbyists over the years and never had one single complaint. I always fall back on the tried and proven methods of carefully measuring and workshop diligence with a final test by a faithful old Oskarblock Vswr meter. I have also published papers that have been peer reviewed in another area of expertise where I achieved a world record result and, up until that point had no computer or hard copy evidence to rely upon as it was all pioneering - it was a remarkable effort that relied almost entirely upon diligent science and scientific practice - or as one of the academics who worked for me once said "suck it and see" lol when the science reached the edge of the world precipice. Those were heady days and long hours of VERY slow and steady progress with tiny incremental steps at every turn. I must admit to having a penchant for those processes when designing and building antennas. So while I stick to what I know as tried and proven I try to keep an open mind . . . .

I would like to see the radiation patterns and the gain when compared to the text books.

Thank you for your input and suggestion.

7.3. Aaron formerly VK4VOX

Hi Long Weekend.

I had a smile from ear to ear while reading your post - thank you so VERY much.

Isn't a shame we don't use the term "swer" in a sentence today ? lmao

How about "basement" - or "top floor" ? or here's one from the archive "going down" lol.

Thank you for the hit of nostalgia - much appreciated.

"we're gone"

Aaron