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6th Jun 2006, 10:56 pm | #21 | |
Dekatron
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Re: Radio earths
Quote:
As a transmitting 'ham' myself I agree with most of what's been said about the importance of good earthing for transmission purposes, having been burned by metalwork 'hot' at RF in the shack! However, for receiving on the MW and SW broadcast bands, every vintage receiver I've ever had has possessed sufficient gain to pull in hoards of stations on a simple untuned aerial wire of under 10 metres in length. This is all that many sets were designed to expect! Phil
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6th Jun 2006, 10:59 pm | #22 | ||
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Re: Radio earths
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This has nothing to do with transmitters but everything to do with impedance matching and maximum RF power transfer between a generator and a load. If your arguments had been that if one was to add tuning components to an antenna system, then it might be more worthwhile to build an all out transmatch / 'antenna tuner' instead of messing with tuning out the reactive component of the ground connection, then I might readily have agreed. That would have made a fair bit of sense. If all you want to receive is one of the local AM stations on MW, then yes, forget the ground connection. Just plug a random piece of wire into the antenna connection. But if you do care enough about signal strength that you might consider using a quarter wave radial or have erected a long, outside wire up high, then things are a bit different IMHO. About a quarter of a century's worth of experience listening on shortwave on many different wavelengths have at least taught your's truly that proper care in construction of an antenna system can make a huge difference for reception. We are not talking picoVolts here, gentlemen. For reception when using a random antenna I frequently use a good transmatch, an old Yeasu FC-901. This unit has a 'Pass Through' setting, so it is easy to switch the unit in and out of the circuit for doing comparisons. The bottom is that I wouldn't make an antenna system for anything but very casual reception without going to some effort to optimize the impedance matching and minimize ground losses when possible. In my previous postings in this thread I made the implicit assumption, perhaps incorrectly, that we were discussing MW and lower SW frequencies, below 10MHz or so. On those frequencies the random end fed wire alone will often have an impedance suitable for a direct connection to a receiver. This is assuming that the ground connection isn't completely off the wall. Best regards Frank N. Last edited by YC-156; 6th Jun 2006 at 11:13 pm. |
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7th Jun 2006, 12:11 am | #23 |
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Re: Radio earths
No matter how good your earth and aerial I'm afraid the main problem is interference radiated by every bit of electronic equipment in your own and neighbouring houses. Much of this interference finds its way into the electrical mains and is radiated from the wiring, or finds its way into your receiver via the mains lead.
When my neighbourhood used to suffer from frequent power cuts it was wonderful to listen to interference free signals on a battery powered comms receiver with a decent aerial. With the posible exception of filament light bulbs I'd say that every bit of modern equipment radiates significant interference. Rant over!
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7th Jun 2006, 10:23 pm | #24 | |
Dekatron
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Re: Radio earths
Quote:
Now... my car is insulated from earth by its rubber tyres. The antenna is tuned against the car metalwork, which acts as 'ground', even though the car is floating with respect to true 'earth' potential. However, that doesn't stop me hearing and contacting amateur radio stations all over Europe, the Middle East and America! It would probably work even better with a low impedance earth connection, I guess. Some of the strongest ham signals emanate from ships and even narrowboats, where the hull is 'earthed' via contact with water. Phil
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8th Jun 2006, 10:19 am | #25 |
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Re: Radio earths
Even though your car is insulated from true earth via the tyres, wouldn't there still be some capacitive coupling between the car underside and the ground? Regarding all the theory discussed in this thread, RF is never an exact science in my experience, there being so many influences on its behaviour from other sources. I often find that a little intuition and experimentation produce better results than following text book preachings to the word.
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8th Jun 2006, 1:35 pm | #26 | |
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Re: Radio earths
Quote:
Your car aerial is probably a quarter wave,and is tuned against the body of the car,this represents the missing half of a dipole,these sort of aerials dont need an earth, I believe most hams use dipoles,qtr wave or some other 2 connection resonant arrays. The earth we are discussing is the one used with a long wire which has to have a ground to complete the circuit.We can get away with this connection for receiving because of stray capacitance,the earth terminal of the receiver is "earthed" via the mains,or,across the transformer windings,and in so doing will be added all the noise and interference that exists with the public mains today.Any other earth connection is often better,even a long lead to a ground spike,but,again best results when this is a very,very short and thick wire to a good ground spike,even though we have a high impedance circuit,you certainly dont want inductance in this connection. One other thing,dont connect a ground spike earth to the earth terminal which is also connected to mains earth.(This is one reason why radios do not have 3 core leads) |
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8th Jun 2006, 5:45 pm | #27 | |
Dekatron
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Re: Radio earths
Quote:
Zak is correct in that most home-based transmitting hams use either a 'balanced' antenna which is isolated from earth, such as a dipole, rather than a long-wire antenna of the 'Marconi' type which is unbalanced and tuned against earth. The reason is that long-wire antennas often cause awful interference to neighbouring TVs when transmitting. My mobile HF antenna is essentially an very short inductively-loaded quarter-wavelength monopole (ie half a dipole) and the vehicle body acts as a ground plane. The ground plane behaves like a mirror, producing a reflection below it of the monopole above it, so you have your dipole again. Whether it causes TVI, I've no idea. I'm usually miles from anywhere or moving along pretty fast when I'm transmitting! I'm not sure whether capacitive coupling between vehicle body and earth is a significant effect at such low frequencies and with such a large spacing between the 'plates'. Phil
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8th Jun 2006, 8:36 pm | #28 | |
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Re: Radio earths
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Sorry, topic drifting. |
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9th Jun 2006, 11:21 am | #29 |
Dekatron
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Re: Radio earths
Some car aerials that you wouldn't expect to work actually perform quite well. I used this configuration http://www.jag-lovers.org/snaps/snap...?id=1117063708 with an HMV 100 for a while. I only gave up with it because I kept skinning my knuckles when gear changing.
Peter. |
9th Jun 2006, 11:27 am | #30 |
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Re: Radio earths
Sorry, I think you need to sign up to see the previous link. Here are the images..
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9th Jun 2006, 9:48 pm | #31 |
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Re: Radio earths
Back in the 1970's I ran my old HMV1124 on a 60' longwire aerial and mains earth. Results were ok but effected badly every evening by interference from TV sets. I then used a seperate earth provided un wittingly by the GPO (for a part line) the results were amazing. Far better pick up on all frequencies and much lower levels of local electrical interference.
When I get some spare time I will be fitting a seperate radio earth as the results are well worth it.
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12th Jun 2006, 1:33 am | #32 | |
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Re: Radio earths
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12th Jun 2006, 1:44 am | #33 | |
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Re: Radio earths
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12th Jun 2006, 9:30 am | #34 |
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Re: Radio earths
Please keep this on topic. It's about signal earths for receivers.
If you want to discuss transmitting aerials, SWR etc please start a new topic.
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30th Jul 2006, 8:49 pm | #35 |
Pentode
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Re: Radio earths
My old Strad had no earth connected and suffered from some hum and was very prone to interference from computers and mobile phones made it go nuts!
So when I replaced the mains lead I installed one with an earth. The results are far better than before. more is picked up than before and there is far less interference that before. Watch out for radio's with a live chassis if you try this though!
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5th Nov 2006, 8:33 pm | #36 |
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Re: Radio earths
I would urge anyone connecting their radio earth terminal to a water or central heating pipe to make sure the earth terminal is isolated with a class-Y capacitor and not to rely on the old wax Hunts effort..even if your radio has a mains transformer.
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6th Nov 2006, 8:50 pm | #37 | |
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Re: Radio earths
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This method usually works ok if you keep the pot watered. Its also fully portable. Regards John. |
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6th Nov 2006, 9:06 pm | #38 |
Octode
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Re: Radio earths
That's an approach I hadn't considered. I assume that the green/yellow colour coding of the wire is essential for this to work effectively? And presumably a John Innes no.3 will also give the best results?
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6th Nov 2006, 9:34 pm | #39 |
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Re: Radio earths
That trick must have been played on hundreds of apprentices over the years. "Go and get the portable earth from the van" is up there with "Go to the stores for a long stand"
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6th Nov 2006, 11:27 pm | #40 |
Nonode
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Re: Radio earths
Don't over water it though, Cacti don't like their feet to be too damp!
Regards,
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