I needed to experiment on region coverage on 10m from our local hill top Repeater

Site I had been listening a lot on the local area CB/PRS frequency 26.450 Mhz to truckies on the clubs DTMF controlled ICR75remote receiver but the problem is its one way comms and they rarely give their geographical position . The HF antenna is a horizontal HF wire dipole, useable on 10m but not ideal, so I decided to make a dedicated 29MHz antenna

I would like good performance, omni directional ( horizontal Radiation) , simple mounting with easy constructional technique so I looked at 2m antennas to get some Ideas, since scaling will take them to any frequency .

I finally settled on a vertical dipole , as it does not require a ground plan and it gives good performance and could be pressed into service as "calibrated device " or reference for testing . For constructional simplicity it will have to be end fed not centre fed (too messy and feedline may distort pattern) .I looked at base feeding as do some CB radio antennas like ringo etc that 50 ohm tap to a resonant tuned circuit at the end of the dipole . There is an issue of a mechanically strong low loss RF insulator to support the end of the dipole and survive the high mountaintop winds ..I didn’t have any pieces of thick strong fibreglass pipe to use .

You can use press fit sliding aluminium pipe at the bottom of the dipole separated by low loss insulating medium eg pvc sleeve inserted into the top of the support pipe and calculate the derived capacitance to resonate the ring inductance that connects the top of the support pole to the base of the halfwave dipole and tap to that , but that may be a mechanical weak point if you cant get a suitable amount of capacitance .

The good ol end fed hertzian dipole was the answer ie the J pole seemed the easiest to construct , gave good performance as the performance analysis software showed, good gain over isotropic and good horizontal performance provided certain constraints are adhered to.. namely

A balun is used to match the coax feed to the balanced tuned lines that feed the base of the halfwave dipole ….( to stop coax radiating RF from the imbalance)

The base of the antenna is not electrically connected to a metal mounting mast supporting the antenna ( the metal mounting mast WILL become part of the antenna and it will radiate induced RF)

3 because of the quarter wave matching stub , the antenna is already 2.4 m above the ground

There is plenty of data on constructional dimensions that can be scaled to 29.6 MHz i.e multiply dimensional lengths by a simple ratio for 2m j poles i.e 146 divided by 29.6 times 146 MHz dimensions gives 10m dimensions and there are also web based calculators to do this as well….. these are all good starting points

It’s lower "U" part was made with 25mm diameter Aluminium tube ,and bent to form a quarter wave 2400mm long each side forming a shorted tuned line spaced at 200mm .
The radiator was 5430 mm long from the top of the matching stub to the tip . the final total length of the dipole plus matching section was 7950mm ( either my dipole was slightly too long or my tuned lines slightly too short )

I used a rather unique way to mount my J pole , I found a length of thick walled 8mm plastic water pipe 200mm in diameter and cut off 2 "rings" 100 mm long and then cut these two rings in half to make four U shaped pieces of plastic. These were bolted to the tuned line as mounting brackets. The centre of the plastic mounts were bored and the whole antenna was bolted to a long piece of dry 100 by 50 wood, this wooden pole will be attached to a strong point at the repeater site .

The half wavelength radiator was made from sliding fit sequentially smaller diameter aluminium to make the 5430mm radiator, screwed together with Stainless steel self tapping screws at right angles as well.

Note: leave the last sliding portion suitably long enough so the length of the radiator Can be fine tuned by sliding the smallest tube in and out ..The other option is to make to base of the 5M radiator long enough to be slid in an out of the top of the 25mm diameter matching stub. This would enable the antenna to be tuned while it is in situ and its easy to reach the telescoping base of the radiator on a step ladder . However you may have to shift the top Plastic pipe mounting bracket at the time of construction , lower down from the top of the tuned line to enable this ..Four length wise slots in the top of the tuned line and a stainless steel hose clamp will facilitate this, but remember the 5M radiator will be a little heavy to slide up and down when on the top of the stepladder !!

I fed the Antenna with coax which was decoupled with a Ferrite toroid and a clip on ferrite ,I would have used just one larger Toroid if I had one, with many more turns through it . Remember you are feeding a BALANCED tuned line , with UNbalanced feeder, so you do need a balun….. without a doubt .

The picture shows the sliding feed point connection made from flat aluminium sheet, between the coax to the parallel lines , there are small bolts to effect a tight immovable connection at the optimal match point . One should really use Stainless Steel components with aluminium to prevent electrolytic corrosion but I didn’t have suitable SS hardware so I used zinc plated bolts and nuts and used a trick I have done many times and is remarkably effective, and that is coating the cleaned metal components with hot wax using a small propane torch ,this will protect for some years

Once every thing is tuned up and im happy with the antenna, I go round with a small blow torch and melt wax on all dissimilar surfaces nuts, bolts and threads ,when the antenna is installed then the important parts will be also wrapped in Denso tape for final weather protection.

I would have used a larger toroid if I had one at hand . This should block transmitted RF passing down the outside of the feed line , and also help stop radiated noise picked up on the outside of the coax of the long feedline getting back into the antenna in receive mode .

The antenna was tuned for minimum SWR ( I used my N5063 VNA) I obtained 1.04 : 1 but an SWR meter is fine ..excite the antenna from your transceiver on low power and just juggle the match point up and down for minimum swr , however you may also have to change the physical length of the radiator to achieve negligible reflected power . I was a little surprised how narrow a bandwith the antenna was, Perhaps I was just used to the 4 MHz sweeps seen at 144 to 148 and maybe as a percentage of operating frequency its no too bad at all ??

As it is said " proof of the pudding will be in the eating" so watch this space and oh yes and I think we might just guy this antenna to survive the high winds !

looks like I may not quite have the correct combination of radiator length and matching point and the antenna looks overly inductive ?? Im not too concerned as the match is good at the frequency of interest and it wont make much difference as the antenna will be used only on one frequency ….

                                                                             Back to home page