Aerials, Feeders, Transmission lines, AMU's and ATUs. Matching.

[Mike. Watterson]

Split from this thread:-

https://www.vintage-radio.net/forum/...d.php?t=157926



I have used a telescopic CB home base (nearly 6m long) clamped to a balcony and also in laybys clamped to the rear towing eye of the car. I removed the loading coil so as to be able to use an Aerial Match box (AKA ATU, Aerial Tuning Unit). It's about the minimal length to be useful vs a maximum practical length without nylon stay cords. So about 1/4 wave at 12.5MHz. It does work with an auto ATU on 80m (3.5MHz to 4MHz), but not sensibly for transmit on 160m (1.8MHz). It will somewhat receive on MW & LW but very poor compared to a typical "Paris Aerial" aka Picture Frame Loop. It will tune and work up to 50MHz and obviously receives on FM Band II, Air and Marine. No use for transmit on 144MHz even with the Auto ATU.

I replaced the slotted hose clamps to lock the sections on the "silver stick" style CB aerial with thumb/butterfly style hose clamps.

I also have a military double discone. The top "cone" uses four standard military whips and the four guys are insulated at the bottom. It uses maybe 4m high mast made out out push together poles like tent poles. It tunes better than the whip, discones being broadband. Just about usable on 2m (highish SWR). Works well enough with an ATU on 3.5MHz to 50MHz, and 7MHz to 20Mhz is feasible without an ATU. Pretty noisy and poor LW, MW, Trawler & 160m. Obviously only for temporary erection on a lawn or field. The canvas carry bag is heavy on its own!

[Mike. Watterson]

Also it's not the ABSOLUTE size that's important but the electrical size, i.e. what fraction of a wavelength.
There are different issues:
1) Aperture. Obviously an array of four dipoles spaced enough, picks up four times the signal. At shorter wavelengths it's easier to use a parabolic dish than an array of aerials to increase signal. A 1/4 wavelength whip aerial on the ground inherently has smaller aperture than a vertically mounted dipole. The whip has got a reflected virtual whip. Spacing four MW aerials to have four times the signal needs about half a kilometre of ground!

2) Efficiency. A folded dipole is efficient if the size 1/2 wave overall. Such an aerial curved to be a "halo" type is very inefficient. A Yagi, dish or some Log periodics can have apparent high efficiency because the beamwidth is narrower. The gain of a dish goes up with the square of the frequency times the square of the diameter! So a 1m dish (fictitious 70% efficient) at 1GHz is 18.8dB. It's 38.8dB at 10 GHz. Increase to 3m and it's 48.8dB. At UHF using a massive array of 16 off Yagis works better. I have a photo of such a thing used on a Tipperary mountain to get UK TV. They had TWO, one pointed to Wales and one pointed at NI. Later there was a licenced microwave link from Cavan (the first private one in Ireland) to carry four channels.

A 60m wire isn't as efficient for MW as a 0.75m wire for VHF radio (traditional whip on car). A typical home vertical wire might be only 10m high. Electrically short, as a 1/4 wave is about 75m.

3) Matching. Certain kinds of aerial can be electrically small, that is smaller than a 1/4 wave. The actual physical size depends on the wavelength for a particular design.

4) Bandwidth. A dipole or whip has wider bandwidth than a Yagi. A Log Periodic or Discone (or double Disccone) can be as wideband as desired, it just gets larger for the lower frequency end and tolerance of parts gets problematic for the high frequency end. Log Periodic and Discones are common for 27MHz to 1.2GHz, but the military, spy agencies and Radio Amateurs can use such for Shortwave. They are rather impractical below 3.5MHz to 7MHz. The discone is usually only vertical polarised. A Cage or barrel aerial is sometimes used MW to 7MHz (not in one band!) to have a broader bandwidth than a wire. A Log periodic, like a Yagi or dipole can be used vertical or horizontal polarised, but at lower frequencies the mast interferes with vertical and mounting can be more awkward.

The halo is evil for VHF radio as it has negative gain and for omni, a vertical whip is best.

A whip needs a ground plane, so on a mast it needs earthed radials. A cute way to do a vertical dipole is a sleeve over the mast, such that ratio of mast dia to sleeve inner is 75 Ohms. The coax goes inside mast and connects outer to sleeve and inner to whip. Thus no groundplane or radials are needed.

5) Matching. A 1/4 is easy. A 5/8ths gives narrower polar pattern thus more gain. A 1/2 wave is very hard to match. If the aerial is shorter than 1/4 wave, then you can add capacitance at the top (radial wires on MW, a disc on UHF), or more inductance at the bottom. As the aerial gets shorter than 1/4 the aperture is less, thus a 1/40th wave (10m long on MW) will pick up very much less radio signal. It will not pickup much less interference that is very local, E near field). Thus one system to reduce interference is a loop or ferrite rod (just a miniaturised loop really) and a short wire, say the the VHF aerial, which will be good at picking up interference. Then if added in to the loop/ferrite signal at the correct phase and level it will reduce interference! You can buy new standalone boxes that do that. Some German valve radios had a switch to use the internal foil dipole (for VHF) to do this on MW & LW.

An Aerial Tuning Unit is actually usually MATCHING, not tuning the aerial. That's why better Amateur and professional HF systems have the ATU (the Match box) at the aerial wire and then use 50 Ohms coax. The matching can be automatic (needs a small transmission signal) or manual. Putting more than 1/10th wave of coax or dual feeder between the aerial and an ATU at the radio can't match the aerial properly.

6) You CAN have a very long wire. That's one that's many wavelengths. Unlike other aerials that pick up at right angles to the element/wire/dipoles, the very long wire only receives ground wave and ALONG the wire.

So important aspects are:
Frequency. That decides what is electrically short.
Size compared to wavelength, not ACTUAL size.
Height off ground.
Feed cable if the aerial doesn't start at the radio.
Matching.

At 138kHz to transmit you need about 2kW into a 25m long wire (vertical or horizontal) to get ONE Watt ERP. Inefficient. If matched, the voltage at the far end is lots of kilovolts.

At VHF you can use 80W and detect the signal bouncing off the moon with four really big yagis. Maybe about 2.5m long beam & 13 elements. Using digital signal processing and correlation.

[G8HQP Dave]

Originally Posted by Mike. Watterson

A folded dipole is efficient if the size 1/2 wave overall. Such an aerial curved to be a "halo" type is very inefficient.

Almost any metal antenna of sufficient size has efficiency approaching 100%. The halo has low gain because it is approximately omnidirectional but it is quite efficient. Maybe you use the term "efficient" to mean something other than power efficiency?

A Yagi, dish or some Log periodics can have apparent high efficiency because the beamwidth is narrower.

Narrow beamwidth means high gain, not high efficiency. The efficiency is probably high anyway, as I said above.

The halo is evil for VHF radio as it has negative gain and for omni, a vertical whip is best.

The halo was commonly used in the past for 2m amateur mobile operation because in the days before channelisation most fixed stations were horizontally polarised.

A whip needs a ground plane, so on a mast it needs earthed radials.

It needs radials or a ground.

As the aerial gets shorter than 1/4 the aperture is less, thus a 1/40th wave (10m long on MW) will pick up very much less radio signal.

Aperture relates to gain, which in turn relates to directivity. The gain of a halfwave dipole is 2.15dBi. The gain of an infinitesimal dipole is 1.5dBi. Something in between will have a gain and therefore an aperture somewhere in between. The problem with a short antenna is matching, not gain/aperture.

It will not pickup much less interference that is very local, E near field).

Local E field pickup will probably vary something like antenna self-capacity, which reduces as it gets shorter.

Putting more than 1/10th wave of coax or dual feeder between the aerial and an ATU at the radio can't match the aerial properly.

That would depend on the matching range of the matcher, and the impedance presented by the antenna. There is no fundamental problem with putting some feeder in the way.

[Mike. Watterson]

Originally Posted by G8HQP Dave

That would depend on the matching range of the matcher, and the impedance presented by the antenna. There is no fundamental problem with putting some feeder in the way.

Yes there is. And the longer the feeder is (in relation to wavelength) the worse the problem is. It's no fun having 200W radiated by a feeder cable mostly inside the building, even though the transmitter SWR is "good".

No, its just the feeder + aerial matched to the radio. It's terrible and can result in pickup or radiation by the feeder. You lose a lot of receive or transmit efficiency. That's why the 1970s the military had complex remote "ATU"s in a big box. The Aerial isn't matched. It just means the transmit SWR looks good or the receive signal and noise has peaked.

This is one of the biggest myths about "ATU"s (Match Boxes). Another myth is a that twin feeder and a balun at the radio + ATU works better than a coax feed. What's happening is that the feeder is the aerial!
I have deliberately used a long outdoor feeder cable to extend an aerial for 160m (1.8MHz) reception. Short the radio end (twin or coax) and the feeder is the aerial and the dipole is a capacity hat.

[Mr 1936]

I agree with G8HQP Dave that there is no Fundamental problem in matching an antennna connected via a length of feeder. It will change the impedance seen at the far end of the feeder, but in principle this can still be matched to any other desired impedance given an appropriate network. There may be practical issues of course, such as the insertion loss of the feeder (which will be higher when terminated at both ends by something other than its characteristic impedance). The matching bandwidth will be narrower the longer the feeder is in wavelengths, which may or may not be important. Unwanted feeder radiation/pickup is caused by common mode currents, due to asymmetry in the system or the lack of an effective balun, again this is a practical shortcoming, not a fundamental one. A remote controlled tuning unit can be a heavy item, and it's not always possible to place it right at an antenna feedpoint, so some length of feeder in between may be unavoidable.

Antenna matching is important for transmitting, because normally the highest possible efficiency is desired. For receiving on long and medium waves, matching is often far less important, and often even with tens of dB of mismatch loss, the external noise delivered to the receiver will still comfortably exceed its internal noise. Matching only really becomes important if your receive antenna is particularly electrically small and/or your receiver is particulary insensitive. I admit that there is some satisfaction in seeing the S meter peak up, but at the end of the day it's the signal to noise ratio which determines how good reception is.

[Mike. Watterson]

Yes, you can absolutely match what the radio sees. But the stupid feeder only is a transmission line (a feeder) if it has the Characteristic impedance at BOTH ends. Otherwise it becomes a radiator, an aerial NOT a transmission line.
This is the commonest mistake made by radio amateurs, "I've got good SWR using the 'ATU' between my feeder and the radio, so the aerial is matched." Unless the feeder is a good bit less than 1/4 wave long you have matched the COMBINATION, NOT the actual aerial and the feeder WILL not be a feeder, it will be the aerial. All you are doing is protecting the PA (finals), you are not matching the aerial.

So, unless the feed line is electrically, short, no the good SWR doesn't mean the Aerial is matched. The Aerial is NOT matched!

Antenna matching is NOT just for the Radio or for transmitting. Yes, "matching" the transmission line + aerial rather than the aerial will let you pick up more signal or transmit full power. However if the line more than 1/10th to 1/8th wave long, the longer it is compared to wavelength, IF the junction of aerial to feed is NOT where it's matched, then the feeder is picking up on receive (more interference recieved) and radiating on transmit (potential interference or damage to your premises).

Do read up on WHY a feeder for an aerial is a transmission line and why a coax won't screen and radiates/picks up and a balanced feed line (300, 450, 600 etc) is an aerial, not a feeder, UNLESS BOTH ENDS are matched with the characteristic impedance.

If the feeder is electrically short or the aerial starts at the back of the ATU (Match box), then a good SWR means the aerial is matched. If the feeder is electrically long, then even perfect SWR ONLY means no reflected signal or mismatch for the RADIO. It's not at all and can't ever match the aerial. Remote ATUs to fit at junction of the aerial and the feed are not a deluxe option or a scam. They mean your feeder is not radiating or picking up!

A feeder is ONLY a feeder if matched at the radio AND the Aerial ends. The Radio end is simple. The Aerial end is simple for VHF, UHF or an aerial with a defined impedance. A G5RV, inverted L or a long wire is unlikely to match at more than one frequency, or maybe at all.
Matching ISN'T just to protect transmitter PAs.

[Mike. Watterson]

Just to re-iterate:
Matching is not just for transmitters.
While not damaging PA/Finals on a transmitter is ONE reason to "Match" and a good one, it's not the only reason.
Even a perfect match at the radio with the world's best ATU (Match box) WILL NEVER match an aerial the far end of a feeder, unless the feeder is electrically short (less than 1/4 Wave and ideally no more than 1/8th wave). If the feeder is long and badly matched to aerial, then matching at the radio simply turns the feeder into an aerial with a capacity hat.

[Mr 1936]

Let's take a balanced "twin" feeder with negligible loss, say 300 ohms characteristic impedance. With a 300 ohm resistive source and a 300 ohm resistive load the voltage and current will be uniform along the feeder. No standing waves, Lovely. At all positions along the feeder, the current in one leg is exactly equal and opposite to the current in the other, so no net radiation.

Now replace the load with say 30 ohms. Depending on frequency, the source will "see" an impedance at the sending end with a resistive part anywhere between 30 ohms and 3000 ohms and a reactive part that swing from inductive to capacitive and back again if the feeder is long enough in wavelengths. At the load end, the load will "see" a source impedance that is always still 300 ohms resistive. So the frequency response will still be flat. There will however be a standing wave on the feeder, with voltage maxima and current maxima alternating in position along its length. But at all positions along the feeder, the current in one leg is still exactly equal and opposite to the current in the other, so no net radiation despite the standing wave.

The cause of feeder radiation with a balanced feeder is unwanted common mode current (in addition to the wanted differential mode), due to the system not being properly balanced. For coax the cause of radiation is unwanted current on the OUTSIDE skin of the screen (in addition to the equal and opposite wanted currents flowing on the inner conductor and the INSIDE of the screen). This assumes the screen is thicker than a few skin depths, which is normally the case at RF. These effects can happen irrespective of whether there is a standing wave on the line or not. They may be exacerbated by the presence of standing waves, but are not caused by them per se.

Various techniques exist (Baluns, Ferrite sleeves etc) to suppress unwanted common mode currents whilst having little or no effect on the wanted differential mode currents, and little or no effect on standing waves.

[Mike. Watterson]

A common mode choke will help reduce feed cable radiation or pickup. You need it at both ends! It also can make it harder to match at the Radio. It STILL doesn't enable matching at the aerial. Only putting the ATU/Matchbox at the aerial end of the feeder does that. That's why they sell them.

[Skywave]

Originally Posted by Mike. Watterson

A feeder is ONLY a feeder if matched at the radio AND the aerial ends. The radio end is simple.

Overall, I agree with that.
You can call the wire / cable / w-h-y that connects the receiving or transmitting end to the aerial proper a 'feeder' by all means, since it is acting as a 'feeder' in the mechanical sense, but will only be a 'feeder' in the electronic sense if it 'sees' it appropriate termination resistance at each end. At the receiving / transmitting end, that can be achieved with an ATU / AMU. Not so easy to replicate that at the aerial end, especially since many aerials change their impedance as the received frequency is changed. The classic way to achieve that match at the aerial end is, of course, the half-wave dipole, but even with that, its effective height above ground must exceed a certain minimum value.

Al.

[Mike. Watterson]

Yes, dipole for the frequency, high enough up and ends far enough from objects. Or the "ATU" between the aerial and the feeder. Then the feeder can connect direct to radio assuming they are the same. Converting 75 / 50 / 300 is simple, no ATU needed. Actually using 75 Ohm coax on 50 Ohms input or 450 Ohms feed on 300 Ohms in (or a 300 to 75 Ohm balun = 2:1 turns) isn't much of a mismatch as long as the aerial matches the feeder or the ATU is between the aerial and the feeder.

[Skywave]

During 1967 - 1969 I was living with my parents and at school: I was an avid SWL. The receiver was an Eddystone 888A; the 'listening room' was my bedroom on the first floor. My aerial was a 110 ft. 'longwire'; the downlead was about only 5 ft. to an elementary C-L-C AMU (called an 'ATU' in those days). The earth was a copper tube 15 ft. long into moist earth connected to the ATU & RX. by a stout, flexible wire. The ground beneath most of the aerial was in a wide and deep hollow, effectively increasing the aerial height above ground quite substantially. (Probably about 40 ft. or more at max.)

O.K., so perhaps the sunspot activity was high, but with that set-up I heard Radio Amateurs on the 3.5, 7, 14, 21 and 28 MHz bands from all over the world. One of my favourites was listening to strong USA signals on their 75 m. band - usually at about 1 a.m. in the morning!

The point I am trying to make is this: the length of the downlead was very short in comparison to the total physical length of the aerial. And the earth connection was important too, especially for 1.8 and 3.5 MHz reception. The 'ATU' was also essential. (I still own that 'ATU', but not the Eddy. 888A, which IMHO, was the best AmRad only receiver that Eddystone ever built.).

Al.

[Mike. Watterson]

Originally Posted by Skywave

The point I am trying to make is this: the length of the downlead was very short in comparison to the total physical length of the aerial.

More importantly the downlead was much shorter than a 1/4 wavelength, so the "ATU" could "see" the aerial, not the feeder + aerial.

[russell_w_b]

On HF transmitters the balanced feeders were often 'pinched in' or 'spread out' so the feeder became part of the matching section. It would start out as (say) 328 Ohms Zo at the sender and, if there was a mismatch over the band, or bands, larger than fitted beneath a two-bob bit on a Smith chart, lengths would be adjusted for a different Zo so the transmitter saw the correct impedance and maximum power transfer was effected.

The impedance at the aerial was thus transformed to the DPZ of the sender. We often ran >2:1 VSWR but the unbalanced power was the one to watch.

[G8HQP Dave]

Originally Posted by Mike. Watterson

Yes there is. And the longer the feeder is (in relation to wavelength) the worse the problem is. It's no fun having 200W radiated by a feeder cable mostly inside the building, even though the transmitter SWR is "good".

You may be confusing impedance matching with common-mode current.

No, its just the feeder + aerial matched to the radio. It's terrible and can result in pickup or radiation by the feeder. You lose a lot of receive or transmit efficiency.

Yes, I think you are confusing impedance matching with common-mode current.

A common mode choke will help reduce feed cable radiation or pickup. You need it at both ends!

No, you don't always need it at both ends. Sometimes both ends can help.

It also can make it harder to match at the Radio.

Only when the antenna happens to be a harder match than antenna+feeder_outer. Normally a good balun makes matching easier.

But the stupid feeder only is a transmission line (a feeder) if it has the Characteristic impedance at BOTH ends. Otherwise it becomes a radiator, an aerial NOT a transmission line.

No! This is a complete untrue! I thought you said you were professionally involved with antennas? Why propagate such myths? You really need to do some reading and get your facts straight, especially as you presume to teach others. People trying to learn about antennas should ignore almost everything in post 83.

If the feeder is long and badly matched to aerial, then matching at the radio simply turns the feeder into an aerial with a capacity hat.

And post 84 too.

Let us be clear, a feeder is a feeder whether or not it is matched. Whether a feeder is also a radiator is not a matter of matching, but a matter of balance: is there a current imbalance, also seen as common-mode current or feeder outer current (in the case of coax). Sadly, many radio amateurs and CBers (and, it seems, some professionals) get this all mixed up. Because common-mode current can affect impedance and SWR the myth has developed that impedance mismatch can cause common-mode current. This is simply untrue.

It is sad that here as well as other websites almost all threads about antennas and transmission lines quickly get polluted by popular myths confidently asserted. Hopefully on here there will be enough people with genuine subject knowledge to correct the errors.

[ms660]

Antenna polarization demo, 56:25 in:

https://www.youtube.com/watch?v=nFtNCPUMoYA

Lawrence.

[Dave757]

Hi,

Whilst agreeing with all that has been said regarding the Halo, I use
one as I am not able to erect an external antenna, and my loft space
in the bungalow is so shallow that no type of beam antenna is possible.
It may have 3db loss over a dipole (facing in the correct direction),- half an
S point, but a vertical would have a 20db loss for horizontally polarised signals.
A strange feature of the 2M halo is that it always receives the PI7CIS beacon
much stronger on the 70cm band than on 2M.
Works ok on 70cm transmit too.

Best 73
Dave G0ELJ