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Old 23rd May 2021, 7:49 pm   #1
G6Tanuki
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Default HF 14/28MHz antenna experimentation.

I've always liked playing with antennas - but have equally always liked to have a 'production' antenna to fall-back to when the test/dev/train antenna just didn't work out.

My 'production' antenna has for the last few years been a Hy-Gain 12AVQ with two radials for each of 14/21/28MHz - but I've always felt it was underachieving.

21MHz is not really a band that excites me - and my 'test' antenna has been a set of telescoping tubes that can extend between 2.4 and 8,5 Metres. In recent times it's been used as a 1/4-wave on 14MHz, with a few 16andabit-foot-long radials strung a few feet above ground in the orchard.

Recently, 10M has 'come alive' with Sp.E and while my 12AVQ works all over Europe with a few tens of watts, to be honest, 10 Watts to a base-loaded badger would probably let me work into Italy.

"Amateur radio Techniques" - the collation of Pat Hawker's "Technical topics" - revealed the circuit in pic.1 intended to allow a quarter-wave 7MHz vertical to work as a half-wave on 14MHz; it occurred to me that I could scale it so a 16.5-foot 1/4-wave on 14MHz would also play on 28MHz as an end0fed half-wave....

My scrapbox had a nice ceramic coil and an Eddystone 'butterfly' capacitor. Cue a day of experimentation and dodging-showers, but I can confirm that 'it works' - to be honest rather better than the 12AVQ.

Next step: finding a nice IP67 box to house the coil/capacitor in [I may be able to fudge things so the capacitor can be replaced by a length of coax] and a relay to switch between bands.
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Old 25th May 2021, 9:14 pm   #2
Radio1950
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Default Re: HF 14/28MHz antenna experimentation.

I dont quite understand how that original circuit can work, either on 20m or 40m?
Does the original designer give any description or clues?
Who is V09N?
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Old 25th May 2021, 9:44 pm   #3
Steve G4WCS
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Default Re: HF 14/28MHz antenna experimentation.

Have a look at IP67 “tupperware” sandwich boxes with the snap lids and silicon seals.
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Old 25th May 2021, 11:47 pm   #4
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Default Re: HF 14/28MHz antenna experimentation.

Quote:
Originally Posted by Radio1950 View Post
I dont quite understand how that original circuit can work, either on 20m or 40m?
Does the original designer give any description or clues?
Who is V09N?
On 40m it’s a quarter wave vertical , coil shorted out, on 20m looks like a L match.
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Old 26th May 2021, 12:43 am   #5
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Default Re: HF 14/28MHz antenna experimentation.

Thanks Nuvistor.

It should have interesting SWRs on each band.

Again, who is V09N?
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Old 26th May 2021, 10:21 am   #6
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Default Re: HF 14/28MHz antenna experimentation.

Probably a Newfoundland SK by now......
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Old 26th May 2021, 1:00 pm   #7
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Default Re: HF 14/28MHz antenna experimentation.

B.H Brunemeier VO9N California

The full article referenced in Radcom April 1973 in G6's post is in Ham Radio magazine Dec 1972 page 52
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Old 26th May 2021, 1:26 pm   #8
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Default Re: HF 14/28MHz antenna experimentation.

That L-match is trying to match the end of a half-wave at its impedance peak. The Q will be very high and the bandwidth narrow, but the thing to beware of is that the resonating capacitor, terminals and connection to the antenna must work at VERY high voltage. It's well on the road to being a Tesla coil

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Old 26th May 2021, 1:44 pm   #9
Jon_G4MDC
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Default Re: HF 14/28MHz antenna experimentation.

Mine works pretty well with just a series inductor.

Let the 1/2 wave go slightly long and it becomes capacitive, with the right series L the match is not too bad and no danger of arcing over. BW could be better most likely but it works.
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Old 1st Jun 2021, 12:27 pm   #10
G6Tanuki
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Default Re: HF 14/28MHz antenna experimentation.

My experiments seem to suggest that it works rather well... I'm using a big Eddystone VHF-transmitter-type butterfly-capacitor with both 'halves' wired in series - that way the RF current doesn't flow through the spindle-bearing/wiper arrangement. It's probably happy to handle a few Kilovolts of RF, looking at the plate-spacing, so I doubt my 100 Watts of SSB will trouble it.

"Works rather well" needs to be confirmed by further tests though before I IP67 it and put it up the mast: during my 28MHz tests there were Sp.E openings meaning you could probably work Italy using a base-loaded badger, and middle of last week there was a Coronal Mass Ejection which led to 14MHz becoming rather numb...

Still, HF antenna experimentation is fun!
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Old 2nd Jun 2021, 1:30 am   #11
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Default Re: HF 14/28MHz antenna experimentation.

Quote:
Originally Posted by Radio1950 View Post
I dont quite understand how that original circuit can work, either on 20m or 40m?
Does the original designer give any description or clues?
Half Wave End Fed Vertical

My initial comment was more about the vagaries of matching, and my suspicions about anything with an end fed half wave radiator.

My field experience is that of installing HF Marine antennas on sailing boats, usually (back stay) slopers and verticals, and remote land HF base stations for ATC Search and Rescue.

I normally found that HF sloping and vertical antennas are easier to match with a length a little shorter than a quarter wave to force the antenna to be resistive and capacitive, and a series adjustable inductor is used to match it up.
This is also the advice that some radio manufacturers give, in that they have designed their Antenna Tuning Units to expect that scenario.

A true quarter wave vertical can be very finicky to “match up” due to height over ground, wet blowing trees, other boat masts, and every conceivable variable the devil can throw at it.
But it can work, just with 50 ohm coax feed, and some SWR.

But I was more curious about that end fed half wave vertical.

I read that whole magazine article, inter alia. Thanks to Terry VK5TM.

I ran some figures of a half wave antenna and a “L” coupler into an RF Simulator program, and found that a match is possible at 28.2 MHz.

I made the assumptions of a 50 ohm source, an antenna end impedance of 2000 +/- J100, ie 2000 ohms resistive, and 5 % (numerical) of reactance, to simulate some proximity and other effects.

A match “at antenna end point” (with no reactance) was SWR 1.01 with optimal L Coupler values of 21pF and 1.52 uH.

Add the reactance of +/- j100 ohms and the coupler easily tuned it out for a similar match, and the coupler L and C values are quite close to original.

Network Loss is 1.4 dB, assuming an inductor Q of 20 and a low loss (air) capacitor.

Throw in 20m of RG213 cable between the transmitter and coupler.
Very similar results are found, although the “sharpness” of coupler tuning is increased a little, and system loss increases slightly.


All above are theoretical of course, do not include any allowance for ground effects, and may not give an indication of radiation efficiency.

After all this, I have to pull my head in about half wave end fed antennas.

I intend to build one soon.

More reading ahead for me. Antennas are almost as good as “messing about in boats”.

Thanks to Tanuki for sharing his experience.
I am no expert on antennas nor RF simulator programs.
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Old 5th Jun 2021, 12:49 pm   #12
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Default Re: HF 14/28MHz antenna experimentation.

The way I'm using it - I have three 14MHz 1/4-wave [8 foot 3 inches for those of you who still understand antique measurements] radials attached to the base - these also serve as guys since the 'base' is on a 20-foot pole bolted to the house using T and K brackets.

The radiator is then adjusted to get a reasonable SWR on 14MHz - I've got it optimised for 14.25MHz and the SWR is below 1.5:1 for 125KHz either side.

This is basically a classic 1/4-wave ground-plane.

On 28MHz the vertical element is essentially a 1/2-wave so presents a high end-impedance and needs to be voltage-fed, hence the L-network. Because the 14MHz quarter-wave radials are now half-wavelengths at 28MHz they too will present a high impedance at the point where they connect to the feedpoint - so they probably play little part in proceedings. I guess that the mast and feeder act as an untuned 'counterpoise'.

With suitable choice of coil-tap and capacitance I can get a good match on 28.4MHz - my ribbed ceramic oil-former wound with 16-gauge wire, and the wide-spaced Eddystone VHF butterfly-capacitor don't seem to show any signs of localised heating or other distress when doing a 100W-of-carrier-for-5-minutes endurance-test.

The issue of 'end-fed half-wave' antennas is discussed in some detail in Les Moxon's "HF Antennas for All Locations" book [pub: RSGB] - he interestingly analyses why the classic "end-fed Zepp" open-wire-fed antenna can't work, how it 'might' sort-of work sometimes - and the benefit of counterpoises [which can actually be quite small, even for a 1/4-wave vertical].

Out of interest, and fearing my measurements might be being perturbed by imbalance-currents on the outside of the feeder, I experimented with winding the coax through a 3-inch-diameter Amidon toroid to form a common-mode choke. No difference to the SWR was noted. Equally, winding ten turns of coax solenoid-style round a piece of grey PVC drainpipe made no difference - I can only conclude that 'outer cable' RF isn't being much of an issue.

Out of note, I've been measuring my radiated field-strength using a battery-powered RF millivoltmeter fed by a 36-inch telescopic whip. I've got this set up around 100 yards away from the antenna, and to ovserve the meter reading I have an old phone set up a foot or so from the meter. A 'video call' to it from my phone means I can watch the meter deflection easily as I experiment, and it takes ages to get through 50 Megabytes of data since the only thing that's changing is the needle-position.
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Old 5th Jun 2021, 2:17 pm   #13
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Default Re: HF 14/28MHz antenna experimentation.

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and to observe the meter reading I have an old phone set up a foot or so from the meter.
The low power way is to use a telescope!
 
Old 5th Jun 2021, 6:38 pm   #14
G6Tanuki
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Default Re: HF 14/28MHz antenna experimentation.

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Originally Posted by merlinmaxwell View Post
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and to observe the meter reading I have an old phone set up a foot or so from the meter.
The low power way is to use a telescope!
Sure, but I do not have anything approaching line-of-sight propagation at 400–700 TeraHertz between my shack and the location of the field-strength-meter.

[In-leaf trees are excellent optical attenuators]
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