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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 2nd Oct 2019, 10:43 pm   #121
Radio Wrangler
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Default Re: Antenna recommendation for 40 and 80M bands

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Originally Posted by Bazz4CQJ View Post
I have never used an end-fed antenna for transmitting, but I assume that to use a GDO or any kind of antenna analyser you would have to inductively couple it in to a circuit with a ground or a counterpoise.
That's true. But leaves you the quandary of are you using the counterpoise to help measure the antenna or are you using the antenna to measure the counterpoise?

Because you can't do a perfect counterpoise, or produce a perfect ground up by the feedpoint of your antenna, the answer has to be 'a bit of both' and it's hard to decide what the proportions are.

The elegant way to evade the problem is to duplicate your antenna in the opposite direction and measure between them. You can logick that each of the two antennae must have the same impedance, so the impedance of one alone must be half of the impedance your measurement gives. Simple, elegant and effective!

It's a general problem-solving technique. Make two identical things and measure one against the other.... you can blame half of the total impairment, value or whatever on each.

It's common to make two identical crystal oscillators and to mix them together to measure their noise characteristics. You get to measure at low frequencies and you can assume half the noise comes from each.

We all started out knowing nothing. Some people merely started learning earlier, but that's all. They have an advantage for only a while if those who come along later are faster and can catch up. There is no progress if the pupil cannot overtake the teacher. Don't worry about it, you're making good progress and you have an interest in the area. Look at athletics. The amateurs run fastest. By the time they turn professional they are getting older and slowing down. An amateur also has the freedom to investigate things that take his fancy. The professional is limited to working in areas the accountants think profitable. Enjoy the freedom!

David
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Old 3rd Oct 2019, 12:11 am   #122
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Default Re: Antenna recommendation for 40 and 80M bands

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Measuring the loss of an ATU is not easy, you need to be able to measure true power into whatever impedance it is transforming. In most ATUs the inductor is the big culprit, and its Q can be spoiled by putting it in too small a box (Whoosh! there went most of the MFJ, Trio, Yaesu and Icom atus)

David
Returning to low-loss ATU's, so if the coil is length L and diameter D, what kind of dimensions does the enclosure have to be to keep a good value of Q?

B
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Old 3rd Oct 2019, 5:33 am   #123
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Default Re: Antenna recommendation for 40 and 80M bands

I've seen papers on the Q of solenoidal inductors and the effects of different sizes of screening cans. I haven't kept references.

For ATUs you want a case at least twice as long as the length of your roller-coaster, and you want space around it of about half its length, or 4 diameters in all directions. you can have capacitors in the clearance space, but envisage the fields and avoid plate metal cutting them. You're trying to avoid eddy currents in anything.

ATU wiring can be lossy, copper tape is useful here, or copper tubing. Skin effect and low impedance circuits say surface area beats cross-sectional area.

I have one roller-coaster where the winding is about 5" in diameter and stationary, the contact rotates and slides, following the coil. The coil is silver-plated flat bar formed into a solenoid shape.

Have a look at the 'Transfer' variable inductor in some classic Marconi transmitters and you'll get a feel for what is needed for a really good ATU.

In general, surplus parts and homebrew is the way to go.

Aluminium cases are better than steel. Eddy currents become reflective more than dissipative.

What you don't want to see is tightly packed stuff in small cases with thin spindly wiring.

David
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Old 3rd Oct 2019, 11:13 am   #124
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Default Re: Antenna recommendation for 40 and 80M bands

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Originally Posted by Bazz4CQJ
I have never used an end-fed antenna for transmitting, but I assume that to use a GDO or any kind of antenna analyser you would have to inductively couple it in to a circuit with a ground or a counterpoise.
Yes, and that is what I said but you still appeared to want to couple it to a single piece of wire; sorry if I misunderstood you. Belief that it is possible to couple to a single piece of wire at one end is astonishingly common, so I gave the 12V PSU analogy to make you think. Sorry for any offence I may have caused. If you realise that currents always have to flow in loops then you are in a minority when it comes to antennas!
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Old 3rd Oct 2019, 11:53 am   #125
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Arrow Re: Antenna recommendation for 40 and 80M bands

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Aluminium cases [for ATUs / AMUs] are better than steel. Eddy currents become reflective more than dissipative.
David
Many decades ago - for receive purposes only - I built a conventional C-L-C ATU in a case made of Perspex to avoid the effects of nearby metal and used it well away from metal objects: it worked well with a 100 ft. 'longwire' plus an earth connection. A few years ago, I rewired it with a coil of larger diameter and used silver-plated wire: the consequent improvement in performance (with same receiver) was noticeable.

Al.
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Old 3rd Oct 2019, 4:34 pm   #126
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Default Re: Antenna recommendation for 40 and 80M bands

I have been monitoring this thread from the start as in a way, with no aerials, like the OP I am starting from scratch.

Way back when in the days when I first got going on HF I was still living with my parents. My budget was limited, aerials were near the end of the garden whilst the 'shack' was a corner of my bedroom at the front of the house. I put up a couple of inverted Vs but as I already knew that simply connecting balanced aerials to coax was a bad idea - and in any case was expensive given the run required and my limited means - balanced feeder was the sensible (and elegant) way to go.

I'd considered other ideas, like verticals and inverted Ls but this meant matching units near the base of the antenna, which would have in turn meant remote control, putting in a ground system which might not be that efficient, plus the dreaded cost of the coax. The problems of earthing discussed earlier in the thread were unknown to me and so never considered.

Feeding the inverted Vs with open wire feeder presented practical problems installation-wise that my parents wouldn't have tolerated but using more manageable and relatively unobtrusive 300ohm ribbon feeder was possible. A bonus was that I had been given some at some point so adding to it wasn't going to be costly. This was solid ribbon but it was around the time that the effects of damp on ribbon were being discussed in Rad Com and I spent many hours painstakingly cutting windows to some dimensions described in Technical Topics.

I built the Z-match coupler described in the RSGB Radio Communication Handbook (4th Ed) - see the attached scans. I had been given a large two-gang wide spaced variable capacitor which did for the split stator unit and I constructed the coils and placed the components precisely as the diagrams. The unit was constructed on an open L-shaped chassis.

I didn't have anything as exotic as an SWR meter. I simply tuned up the all-valve transmitter and adjusted the Z-match until I got the correct indications on the transmitter meters.

How efficient it all was I've little idea. However, I made plenty of contacts including maintaining regular skeds with the US on 20 and 15 metres and had lots of enjoyment.

I still have the unit and am considering a stealthy 40m dipole fed with open wire feeder (different house and no parents to upset!).
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Old 3rd Oct 2019, 5:35 pm   #127
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Default Re: Antenna recommendation for 40 and 80M bands

Yup, that's the one.

Built carefully, it is a nice, low-loss ATU. It doesn't cover as extreme a range of impedances as some tuners do, but it covers enough. Extreme impedances are inevitably accessed via high-Q ( = increased loss) transforming circuits.

It also does the balanced transformation for you for free, and avoids the losses in baluns.

There's a commercially made version (Decca branded KW) sitting by my right elbow connected between an ICOM IC7700 and the open wire feeder to the inverted V over the house.

David
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Old 3rd Oct 2019, 10:21 pm   #128
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Default Re: Antenna recommendation for 40 and 80M bands

1:1 balun arrived, and when I inserted between the wire and coax, reception got real quiet on all bands.

And tuned bandwidth has increased and is wider than before.
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Old 5th Oct 2019, 4:52 pm   #129
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Default Re: Antenna recommendation for 40 and 80M bands

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1:1 balun arrived, and when I inserted between the wire and coax, reception got real quiet on all bands. And tuned bandwidth has increased and is wider than before.
So, looking at your post #99, am I right in saying you've now got an inverted V, cut for resonance on 80m, with a 1:1 balun feeding coax to the shack, and I'm not sure about what you finally did re an ATU?

My reason for asking is that I have men with a big chainsaw arriving on the 17th to do some major tree work (you have to book these guys months in advance in leafy Oxfordshire!) and thereby creating a new opportunity to put up an 80m inverted V. It will be Christmas come early .

(trees are not being felled just to make room for an antenna, but in part, to keep a reasonable (he's paying 50%) neighbour happy).

B
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Old 5th Oct 2019, 10:47 pm   #130
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Default Re: Antenna recommendation for 40 and 80M bands

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Originally Posted by Bazz4CQJ View Post
So, looking at your post #99, am I right in saying you've now got an inverted V, cut for resonance on 80m, with a 1:1 balun feeding coax to the shack, and I'm not sure about what you finally did re an ATU?

My reason for asking is that I have men with a big chainsaw arriving on the 17th to do some major tree work (you have to book these guys months in advance in leafy Oxfordshire!) and thereby creating a new opportunity to put up an 80m inverted V. It will be Christmas come early .

(trees are not being felled just to make room for an antenna, but in part, to keep a reasonable (he's paying 50%) neighbour happy).

B
Yes, the inverted V was working ok on 80m and 20m, but not good on 40m.
I was going to add another wire cut for 40m band making it into a fan dipole, but I thought I would try something else.

I have taken it down, and set up a wire loop. It is total length of 40m long, and the wire comes directly into the ATU balanced ant. input post.
I have acquired another ATU with unbalanced and balanced input now.

It has been working OK on most bands. Feels very efficient on TX, and RX is also up by S2-3 points than all other antennas I have tried so far.
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