An interesting write-up on half-wave aerials.

[Skywave]

This may have appeared on this forum before - but it is the first time I've read it anywhere.

http://www.qsl.net/arrlsb/Digest/Pag...ntennas03.html

Many a provocative thought in this - quite likely to generate a few comments. Nevertheless, a good read - but not to be read if you only have limited time - it's quite detailed and thorough.

Al. / Jan. 2

[Bazz4CQJ]

Worth reading, as although it starts off on "Half Wave Antennas", it actually goes on to cover a diverse range of subjects about the downstream world after the Tx. I'm surprised by the claim that 70% of us reject the idea of using an ATU; surely not true ?

B

[Skywave]

Originally Posted by Bazz4CQJ

I'm surprised by the claim that 70% of us reject the idea of using an ATU: surely not true ?

I suspect that that remark was slanted towards Amateur Radio enthusiasts, predominately in the U.S.A.

But to summarise: what I drew, in conclusion from that article, was that the old school of "put up a convenient length of wire, as long and as high as you can get it, couple it to your TX with a pi-coupler (or any variation of a Z-matching device), and off you go", is as valid now as then.

Al.

[GW3OQK Andrew]

Exactly Al. My doublet with 450 ohm feeder along with a Z Match does the job reccomended in the article's conclusion,

I have done quite a few receive comparisons with instant switch-over from doublet to inverted L. (Strength, not s/noise ratio) The inv L signals are usually strongest. Theeoretically on 20m the doublet has 4 lobes of maximum at each quadrant that should be best for USA for example, but the Inv L still seems best. I have been planning on a simpler tuner than the Z match just in case its inefficient.

73 HNY
Andrew

For my transmit antenna I have a 10m vertical with an SGC autotuner at the base, lots of earth rods too. Works a treat.

[Bazz4CQJ]

Originally Posted by Skywave

couple it to your TX with a pi-coupler (or any variation of a Z-matching device), and off you go", is as valid now as then.

But just to be doubly clear, we're advocating the use of an effective ATU of any kind, not specifically a "Z-match" which some would regard as being specific to a transformer-based ATU design? Curiously, I'm just in the process of rebuilding a series/parallel capacitor transmatch. I originally threw it together in a plastic cake box and it worked so well, but looked so ugly. Sod's law demands it will be less good when neatly built inside a nice metal chassis, even with good spacing of all the components .

B

[G6Tanuki]

An interesting article - but he misses the essential difference between an antenna *tuner* and an antenna *matcher*.

An antenna tuner brings an antenna to resonance by adding L or C or some combination of the above to the radiating elements. The aim is that by bringing the antenna to resonance it presents an ideal, resistive impedance to the feeder.

An antenna matcher is attached to an antenna/feeder which presents a complex impedance at the operating frequency, and uses L/C/transformer-action to con the attached transceiver into thinking it is working into a non-reactive load. It doesn't actually change the resonant frequency of the antenna.

To my way of thinking - a tuner is directly-attached to the antenna and directly alters the antenna's resonant characteristics. The "Screwdriver" type mobile antenna is a good example of this, similarly the remotely controlled motor-driven high-voltage-capacitor used with transmitting loops.

A matcher sits at the transmitter-end of the feeder.


Me? I'm currently using a 10-metre dipole (which is resonant at 28.3MHz according to my GDO) and for the LF bands a 100.mumble-foot longwire with a Redifon roller-coaster which adds series-inductance. In the latter case I consider it as a tuner because I seek resonance with the GDO then switch to transmit.

[Skywave]

Originally Posted by GW3OQK Andrew

. . . .the Inv. L still seems best. I have been planning on a simpler tuner than the Z match just in case its inefficient.

In this location, I've been using a 'T' 'long-wire' aerial (60 ft. long, 25 ft. high) for ages for listening purposes only and have experimented with various ATUs, plus good earthing arrangements, over the years for use with various receivers: not all have the same input impedance, of course. Overall, the use of an ATU (or AMU if you prefer) has always been beneficial to increasing received signal strength - and occasionally the removal of spurious signals. The ATU / AMU acts as a matching device, of course: the object being to tune out reactance and match the resistive component. The difference in the performance of various ATUs / AMUs has been considerable.

However, having said all that, in a few days time, I will be leaving this location for ever. Fortunately, the new home affords much more real estate and high points for aerial experimentation - so it's back to the drawing board. A balanced doublet is favoured, since the shack will be in an upstairs room, but there will be problems arranging for a balanced feed-line to leave that aerial at the appropriate angle (90° ideally) on account of the orientation of the garden and the house and also to enter the shack without seriously twisting the feed-line to an undesirable extent.
But all that as may be: spring is on its way - well-known as 'antenna time' - fun should be assured!!

Al. / Jan. 3, '15 //

[Skywave]

Originally Posted by G6Tanuki

An interesting article - but he misses the essential difference between an antenna *tuner* and an antenna *matcher*. (And everything written after that which justifies and clarifies that statement).

Good point: agreed in total.

Al.

[G8HQP Dave]

Originally Posted by G6Tanuki

An interesting article - but he misses the essential difference between an antenna *tuner* and an antenna *matcher*.

An antenna tuner brings an antenna to resonance by adding L or C or some combination of the above to the radiating elements. The aim is that by bringing the antenna to resonance it presents an ideal, resistive impedance to the feeder.

An antenna matcher is attached to an antenna/feeder which presents a complex impedance at the operating frequency, and uses L/C/transformer-action to con the attached transceiver into thinking it is working into a non-reactive load. It doesn't actually change the resonant frequency of the antenna.

There is no essential difference. In both cases a given antenna has its feed impedance modified in order to provide the impedance preferred by a transmitter. Which end of the feedline this takes place may affect feedline losses but that is all. In neither case is the antenna actually changed, and in neither case is the transmitter conned as it actually does see a non-reactive load.

You can lose the artificial distinction between an antenna 'tuner' and an antenna 'matcher' simply by shrinking the transmission line to zero (or using an ideal lossless one). You then have two things which "essentially" different, yet they are the same thing. Two essentially different things cannot be continuously transformed into each other. Alternatively, put the matching/tuning in the middle of the line. What "essentially" changes as you vary its position from one end to the other?

[Skywave]

Originally Posted by G8HQP Dave

There is no essential difference. In both cases a given antenna has its feed impedance modified in order to provide the impedance preferred by a transmitter.

Really? Doesn't sound quite right to me. A theoretical and perfect half-wave dipole on transmit will present a load resistance of 73 Ohms. If a feeder of 73 Ohms is used to connect that dipole to an energising source and that source is not 73 Ohms, a mismatch will occur at the energising source-to-feeder junction - not at the load, so the antenna's feed impedance will still remain at 73 Ohms. But the energising source will now require an impedance transformation, so that that source 'sees' 73 Ohms, i.e. the Zo of the feeder (73 Ohms) then the antenna load: 73 Ohms - which hasn't changed.

So, am I missing something in that line of thought?

Al.

[G8HQP Dave]

Nothing fundamentally/essentially changes. Obviously you will want to present the correct load to a feeder (unless you are deliberately exploiting feeder impedance effects, such as a G5RV). A matching device at the antenna end does not change the antenna, unless you redefine the word "antenna" to include the matching device too.

To feed a 73R antenna from a transmitter which expects to see a 50R load you can use a matcher at the antenna end and a 50R cable, or a matcher at the transmitter end and a 73R cable. There is no fundamental difference between these. You could even use a mixture of the two setups: 73R cable to matcher then 50R cable to transmittter.

The real issue is about reactive matching. Some people think adding, say, extra lumped inductance at an antenna feedpoint "tunes" the antenna. Others know that it is simply compensating for excessive capacitive reactance at the feed point. What we call it doesn't matter too much as the equations are the same. Problems arise when people attach too much meaning to a loose description or try to argue that two essentially identical situations are somehow fundamentally different.

[GMB]

Yes, a good article although I think he didn't dwell so much on the main problem with funny bits of wire as aerials, which is that you can get rather strange radiation patterns. He also didn't perhaps warn that massive standing waves can result in a ball of plasma appearing where you didn't want it.

I use a random bit of wire and a big tuner. My setup is not very efficient, but only because it is much too near to the ground - although this does simplify the radiation pattern as it all goes straight upwards.

I also have a device that hardly any hams seem to use - a balance meter.

By the way, a couple of my definitions for you:
"Aerial" is the bit of wire that looks like it does the radiating.
"Antenna" is the bits of wire, ground, your house, your body, etc. that actually does the radiating.

[G6Tanuki]

A matcher does not need to be resonant or frequency-selective: a 4:1 ferrite-core transformer used at the feedpoint of a folded dipole will transform the impedance to 75-ohms - ideal for feeding by low-impedance feeder (which can be coax or balanced-twin).. It does not affect the resonant frequency of the antenna so it is not a 'tuner'.

Same goes for things like delta- or gamma-matches on dipoles.


[note: a transformer at the dipole feedpoint is not necessarily a 'balun' either!]

[G6Tanuki]

Originally Posted by GMB

By the way, a couple of my definitions for you:
"Aerial" is the bit of wire that looks like it does the radiating.
"Antenna" is the bits of wire, ground, your house, your body, etc. that actually does the radiating.

My favourite one is "Power Supply: A broadband matching device that optimally couples RF energy on the mains to just where it is least wanted in a receiver".

[G8HQP Dave]

Originally Posted by GMB

By the way, a couple of my definitions for you:
"Aerial" is the bit of wire that looks like it does the radiating.
"Antenna" is the bits of wire, ground, your house, your body, etc. that actually does the radiating.

Very useful definitions. Understanding this distinction would resolve many arguments about miracle small antennas etc.

[GMB]

I forgot another important definition:
"Earth" - a mythical entity with large cult following in the ham community where many believe that it exists in their gardens or even in their water pipes.

[Skywave]

Originally Posted by GMB

By the way, a couple of my definitions for you:
"Aerial" is the bit of wire that looks like it does the radiating.
"Antenna" is the bits of wire, ground, your house, your body, etc. that actually does the radiating.

Really? News to me. I've always understood the two words to mean the exactly same thing: 'aerial' being a British term; 'antenna' being an American term.

Al.

[G8HQP Dave]

I think GMB was being both helpful and amusing. 'Official' definitions may differ from his.

[James Duncan]

Yes
All quite interesting but like all beliefs re antennas the replies are full of differing opinions but the laws of physics cannot be changed.
It is a subject that will live on mostly due to the fact that the same antenna will give differing results depending on location etc.

The main problem is that there is no such thing as an “antenna tuner" just does not exist.
The device provides capacitance and or inductance to match the feeder to the rig and just fools the rig into seeing a “tuned antenna"
The only method of correctly “tuning an Antenna" is by getting the element lengths correct for the band required, most people would not go wrong with creating a correctly cut fan dipole, cheap and very effective.

Claims for an all band low SWR antenna should be viewed with suspicion; these can only work with the addition of a matching unit “now wrongly called an antenna tuner”
MM0HDW (QRZ.COM)