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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 4th Jun 2006, 8:21 pm   #1
peter_scott
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Default Radio earths

One of my fellow retired work colleages just related his experiences to me about resurrecting an old radio.

This may be obvious to you RF guys but without giving it much thought I had always assumed that a domestic ring main earth would serve as a radio earth but my friend found that the 30 or so feet of earth cable between his radio and the earthing rod was clearly acting as an aerial and thus reducing his effective aerial signal by common mode.

After burying an old stainless steel sink in the garden close to the radio he
was very pleasantly surprised by the substantial increase in signal strength
which brought the weaker signals within range of agc control.

Peter.
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Old 4th Jun 2006, 8:34 pm   #2
AndiiT
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Default Re: Radio earths

Hi,
Whilst not being a true "RF guy" I believe that the domestic ring main earth carries too much interference to act as an effective RF (or signalling) earth.

Andrew
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Old 4th Jun 2006, 9:45 pm   #3
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Default Re: Radio earths

It's always preferable to have a seperate RF earth, as the mains earth is prone to interference. Getting teh earth impedance down can be a problem though!

Jim.
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Old 4th Jun 2006, 9:48 pm   #4
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Default Re: Radio earths

Hi,

The thing about the "Earth" (i.e. the planet) is that it behaves like a very, very big capacitor. You can put any amount of electrical power into it and its potential will not move one little bit. If you earth a radio receiver via any significant length of wire it will have its own inductance and hence impedance and the remote end of it will be anywhere but "earth".

You are almost exactly right to identify it as a common mode effect. That earth wire will be picking up a pretty good signal that will be subtracted from the aerial signal.

So keep those earth wires as thick and as short as possible, and water your earth spike in dry weather

On a similar topic, has anyone solved the problem of how to get a good earth in a loft room, where my workshop happens to be?

Cheers, Richard
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Old 4th Jun 2006, 9:55 pm   #5
maitiustandun
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Default Re: Radio earths

hi dickie , how about a lightning conductor strap regards maitiu.
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Old 5th Jun 2006, 6:39 am   #6
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Default Re: Radio earths

Quote:
Originally Posted by Dickie
On a similar topic, has anyone solved the problem of how to get a good earth in a loft room, where my workshop happens to be?
Yep, google for 'tuned counterpoise'.

The idea is that you make a floating quarter wavelength radial (or as long as you can make it). In all likelihood you will not hit the right length to get a proper RF ground. So the idea is to add an 'antenna tuner' in series with the ground connection, allowing you to tune out any unwanted reactances.

There are commercial versions of such a gadget available (MFJ stuff is junk...) but a 'radial tuner' is very easy to make yourself, especially for receiving applications. You just need a variable capacitor and a coil, both connected in series. The trick is of course to figure out what size of inductance and capacitance you will need for a given wire and frequency.

Best regards

Frank N.

Edit: Forgot to mention: For MW/LW you can in a similar fashion 'tune' any overly long and ever so lousy RF ground connection for near perfect performance. Just remember to add a beefy RF choke across any capacitor if the ground connection serves a safety purpose as well (Ie. don't break the DC connection.)

Last edited by YC-156; 5th Jun 2006 at 6:47 am.
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Old 5th Jun 2006, 4:24 pm   #7
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Default Re: Radio earths

Series tuning a long earth wire is a good solution. If the wire is a little more than a quarter wavelength at the fequency of interest, it will have inductive reactance which is easy to tune out with a series variable cap.

An excellent low inductance conductor to install is houshold copper water pipe, the larger the better, with the usual soldered joints. Continue the pipe into the soil for a goodly distance, say 30 feet, and 2 to 3 feet down. Cap the ends of the pipe and install a tee where the pipe daylights, with a connection for your garden hose. Drill about a one eigth inch hole every foot in the underground section.

Now when the soil dries out, you can connect your garden hose to your ground wire, turn on the water, and after about an hour you will be back to a low resistance ground.
John.
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Old 5th Jun 2006, 7:59 pm   #8
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Default Re: Radio earths

Thanks for those ideas, it hadn't really occured to me you could tune out the inductance. I wonder how sharp the tuning would be on medium wave.

Thinking a bit, a quarter wavelength is going to be about 75 m. So my earth lead is going to be longer than my "long-wire" aerial. And it sounds as if I could combine it with a garden irrigation system. I think experimentation will have to wait until our hosepipe ban is lifted!

Richard
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Old 5th Jun 2006, 8:19 pm   #9
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Default Re: Radio earths

Quote:
Originally Posted by Dickie
Thinking a bit, a quarter wavelength is going to be about 75 m.
The point is that the ground connection doesn't have to be anywhere near that long to work well.

If the actual reactance looks like it is shorter, it will be capacitive. Thus you would of course expect it to require an inductance to tune it. But unless you have a variometer available, it might be easier to experiment with a fixed coil with some taps on it instead.

You (probably) start with a capacitive ground connection. Add enough inductance such that the combination becomes inductive. Then add a variable capacitor, which is easy to find, to tune the whole thing to resonance.

The tuning probably won't be very sharp, since you will in all likelihood have fairly high ground losses, thus low Q.

I would just connect anything in the vicinity made from metal, except for the AC mains wiring of course, to a common ground point. Then tune that using the series gizmo just mentioned and make the connections to the radios to that.

Central heating pipes are a recommended domestic RF ground for low power applications if made from metal.

Best regards

Frank N.
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Old 5th Jun 2006, 8:28 pm   #10
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Default Re: Radio earths

Gee this thread reminds me of my very early days spent playing with AM on 160m. I used to have a counterpoise earth with a light bulb and variable capacitor in series. Key up, tune for maximum glow in the bulb then short the bulb out. I used to get fantastic reports with that
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Old 5th Jun 2006, 8:32 pm   #11
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Default Re: Radio earths

An excellent low inductance conductor to install is houshold copper water pipe, the larger the better, with the usual soldered joints. Continue the pipe into the soil for a goodly distance, say 30 feet, and 2 to 3 feet down. Cap the ends of the pipe and install a tee where the pipe daylights, with a connection for your garden hose. Drill about a one eigth inch hole every foot in the underground section.


Thinking along these lines could you just connect your earth wire to the outside tap? or is that a silly idea?
Regards
Paul.
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Old 5th Jun 2006, 8:40 pm   #12
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Default Re: Radio earths

Quote:
Originally Posted by JHGibson
Series tuning a long earth wire is a good solution. If the wire is a little more than a quarter wavelength at the fequency of interest, it will have inductive reactance which is easy to tune out with a series variable cap.

An excellent low inductance conductor to install is houshold copper water pipe, the larger the better, with the usual soldered joints. Continue the pipe into the soil for a goodly distance, say 30 feet, and 2 to 3 feet down. Cap the ends of the pipe and install a tee where the pipe daylights, with a connection for your garden hose. Drill about a one eigth inch hole every foot in the underground section.

Now when the soil dries out, you can connect your garden hose to your ground wire, turn on the water, and after about an hour you will be back to a low resistance ground.
John.
Are you saying that if the lead is shorter than a quarter wavelength then it will be capacitive,and a series inductor is needed?
Ideally the earth wire should have no inductance.If you use an antenna tuner,does this tune out both aerial and earth? after all its the same path,(if we neglect the strays from chassis to mains)
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Old 6th Jun 2006, 2:39 am   #13
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Default Re: Radio earths

It is better to have the ground lead slightly more than a quarter wavelength thus making it have inductive reactance, because it is much simpler to tune out the inductance with a variable cap which has a high Q, than have the lead shorter and need a variable inductor, which is hard to find with a decent Q.

An antenna tuner would work but that is more complicated.

John.
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Old 6th Jun 2006, 9:41 am   #14
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Default Re: Radio earths

Quote:
Originally Posted by JHGibson
It is better to have the ground lead slightly more than a quarter wavelength thus making it have inductive reactance, because it is much simpler to tune out the inductance with a variable cap which has a high Q, than have the lead shorter and need a variable inductor, which is hard to find with a decent Q.
No, it isn't. A single, or a few, quarter wavelength radials are pointless and a waste of good copper.

For a given amount/length of radial copper wire, it is usually better to have more shorter ones than a few long pieces of wire. A radial close to the ground obviously couple strongly to its immediate surroundings, thus the RF current never reaches the far end of a single 1/4 lambda radial.

You need to have an extraordinarily large number of radials, fanning out in a circle from the ground point, to make 1/4 lambda radials worthwhile. Ie. the total ground losses have to be very low before there is anything to gain from using such lengths.

The following table holds for radial wires being very close to or on/in the ground and shows the optimum length in lambda versus number of radials:

4 : 0.10
12 : 0.15
24 : 0.25
48 : 0.35
96 : 0.45
120: 0.50

Source: "Low-Band DXing" by John Devoldere, ON4UN, first edition (1988), page 2-23.

So if you only use 4 radials, the RF currents never reach beyond 0.1 lambda if there is strong coupling to the ground.

A different way of thinking about this is that radials close to the ground or other foreign objects (like a house) aren't resonant in the usual sense and their behaviour cannot be compared to wires in free space. Additionally, for simple ground systems using only a handful of wires or less, the ground loss resistance is so large that the losses in an even halfway decent tuning coil are completely irrelevant to the overall performance of the system.

Best regards

Frank N.

Last edited by YC-156; 6th Jun 2006 at 9:50 am.
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Old 6th Jun 2006, 11:55 am   #15
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Default Re: Radio earths

I'm no expert on these things but if you used a balanced aerial, wouldn't that negate the need for an earth? Thinking about it, one leg of the dipole is going to act as a counterpoise? I would imagine it's still necessary to maintain a true, electrical earth as well.
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Old 6th Jun 2006, 12:28 pm   #16
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Default Re: Radio earths

Quote:
I'm no expert on these things but if you used a balanced aerial, wouldn't that negate the need for an earth?
Exactly what I was thinking - it seems earths are a pain. I presume that the radio is not expecting a balanced signal so a balun is desired.

Or what about a loop aerial?
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Old 6th Jun 2006, 1:18 pm   #17
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Default Re: Radio earths

Balanced aerials such as dipoles are a good idea. However of all the radios I own only the AR88 has provision for a true balanced input.

All the other radios have their signal earth connected to chassis earth, so you're using mains earth as a signal earth whether you like it or not.
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Old 6th Jun 2006, 3:49 pm   #18
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Default Re: Radio earths

Most of this disscussion is really about how to make a perfect earth for a transmitter. Receivers are much less fussy. A heavy wire to a plate or rod in damp soil, or to a cold water tap is usually sufficient.
John.
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Old 6th Jun 2006, 5:28 pm   #19
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Default Re: Radio earths

I would agree with you JH. The theory is probably the same for transmission and reception, but unless you are after that last picovolt of DX, for receivers it probably isn't that critical. The balanced antenna gets round most of these problems and I suppose you could use a balun to match the unbalanced RX input to a balanced dipole, but for normal domestic listening I have never needed to go to these lengths. I suppose much of the discussion stems from the eternal quest for the perfect antenna and earth system. (I'm guilty of this and I suspect that most of the other amateur radio bods out there have been the same at some time)

Biggles.
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Old 6th Jun 2006, 5:51 pm   #20
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Default Re: Radio earths

My thoughts are as follows:

What is required for efficient broadcast radio receiver reception follows the same requirement for efficient transmission where Radio Frequency (RF) energy must be efficiently matched to free space for good propagation and radio frequency safety as well as electrical safety is required. I like to think of the 2 earths required:

1. An RF (Signal) earth.

2. An electrical power earth (the mains power earth).

For broadcast receivers as with transmitting equipment the end feed Marconi aerial (end feed or directly feed or long wire) was most popular. Dipole, multi wire (multi Band) inverted Vee, trapped dipoles etc. are favourite for High Frequency Amateur and commercial radio operations with an antenna tuning unit to match the impedance of the radio equipment to the arial and the aerial length matches the band of wave lengths to be received and transmitted.

For good broadcast receiver reception using a long wire aerial the earth should be as short as possible for the reasons outlined in the above threads (Common Mode). The signal earth can act as a long wire ((earth) Arial) that detracts from the radio frequency received by the long wire aerial. The receiver front end will tune the radio frequency required for reception received by the aerial long wire but the earth length is not tuned by the same components hence the discussion regarding the use of Radio Frequency earth tuning using an (antenna) tuning device in the earth lead in an attempt to match the RF earth to the earth impedance so that the receiver chassis is at or near to earth potential at the desired reception frequency. Some of the discussion is about the ways to achieve a good signal earth and basically the more conductivity there is between the receiver signal earth and the actual buried earth by connection to buried copper rods, grids, water tanks, garden sprinkling systems buried stainless steel sinks etc; by the highest capacity earth conducting material available: lightning conductors, copper soldered water pipes (better than the older half inch metal screwed pipe where the screw threads are made water tight by tallow and hemp increasing electrical resistance/impedance), central heating water systems (not plastic) etc, the better.

For transmitters and receivers (transceivers) above ground the signals earth system above can be enhanced by the use of a radial ground wire cut to a quarter wavelength of the centre frequency of each of the bands to be tuned. This ground wire acts as a counterpoise and makes a signal radio frequency earth resonate at the desired radio reception frequency with in the frequency band. It is be possible to use a single wire and an earth tuner to obtain the same effect but for broadcast reception a number of different length of radial earth wires is easier and cheaper for each of the MW, LW and SW bands. One end of each of the insulated radial ground wires is connected to the radio receiver signal earth and the other is run in a horizontal plane in as straight a line as possible, not necessarily under the aerial wire, along skirting boards, and for the lower frequency (longer wavelengths) can be run outside the building along walls and across the garden. It is not buried and the end is not connected to earth so that RF resonance is maintained and the receiver end is at low impedance.

In short, the requirement for transmission is the same as for reception: the RF radial earth wire tunes the radio frequency signal earth and is used in conjunction with the (mechanical) earth system to bring the receiver chassis to a low potential above the ground (earth) potential enhancing the long wire aerial signal tuned by the radio frequency front end tuned circuits.

In practice, for broadcast reception of MW, LW and SW where transmission of radio frequency power is not an issue a good earth spike and a long wire aerial cut to a suitable length is sufficient. (Counterpoise earth systems are more practical at higher frequencies because of their shorter lengths). However, for those extra microvolts of signal for distant high frequency (SW) transmitting stations you could try a radial earth wire as well as the (mechanical) earth.

Best regards to all

Pete

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