17th Jul 2019, 8:39 am | #21 |
Hexode
Join Date: Nov 2018
Location: Greater Manchester
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Re: Aerial for old valve sets.
Hi Mike very ingenious construction method and a great result. Yesterday whilst on top of my shed (trimming the Hawthorn bushes) I attached an insulated connection point to one of the poles which support my 5RV. Now need a further support under the gutter before running a long wire all round the garden. Looking forward to the results.
Lynton
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17th Jul 2019, 9:34 am | #22 |
Dekatron
Join Date: Apr 2011
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Re: Aerial for old valve sets.
The vertical part of a wire antenna is normally the most responsive, the transmissions on MW and LW are vertically polarised…..unfortunately so is a lot of local interference, generally the higher the antenna the better, another possibility is a tuned loop or a screened loop, both of which ideally need to be rotatable because loops are directional.
In the old days a typical proper installation for MW and LW would consist of a vertical whip mounted on say a chimney, it being connected to the receiver using a transformer and a twin screened downlead with another transformer at the receiver end. Lawrence. |
17th Jul 2019, 9:48 am | #23 |
Tetrode
Join Date: Sep 2018
Location: East Grinstead, West Sussex, UK.
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Re: Aerial for old valve sets.
I find this advice concerning a vertical whip, twin screened downlead and transformers very interesting and something I would like to try (but it won't be me up the ladder).
Lawrence, have you any guidance on the various component specifications and method of terminating the screened downlead? |
17th Jul 2019, 10:24 am | #24 |
Dekatron
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Re: Aerial for old valve sets.
No specific info as such for that interference reduction system but a general layout is shown in the Radio and Television Engineers Reference Book (Hawker & Pannett) 21-8 & 21-9 (shown below)
Lawrence. |
17th Jul 2019, 10:55 am | #25 |
Heptode
Join Date: Dec 2014
Location: Limerick, Ireland.
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Re: Aerial for old valve sets.
A vertical with screened feed was rare. Most people used a longwire, often a clothes line. Loops on picture or photo frames common in France.
The feed would usually have been coax. A four to one transformer at the radio changes 75 Ohm to 1200 Ohm. Many radio aerial inputs around that value. KB sold the Rejectostat, basically two transformers. You can even have 30 turn loop in the attic with maybe 6 turns to a coax and then a 3:1 or 4:1 transformer wound on a glass jar or card tube about 2.5" / 6cm. A ferrite rod needs the same number of turns (about 500:125 for MW), it just takes less space and acts as an aerial unless the ferrite is short. Any screen needs to be about twice a coil diameter or it's a shorted turn, unless it has slot at right angles to the winding. I've a 32cm x 17cm loop (inside dimensions of frame), about 30 turns connected with about 1.5m of twisted pair to the AE socket. It's a photoframe standing vertical, about 4cm deep outside. I have a slide switch and a "resistor" style 2.5mH inductor switched in for LW. Curiously works on SW better than a wire as there is too much interference on anything other than a loop. The TV coax connector (Belling lee) was actually developed for MW radio in the 1930s. It's seriously inferior to F-Connector or BNC. Edit: Impedance ratio is the voltage or turns ratio squared. |
17th Jul 2019, 11:54 am | #26 |
Tetrode
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Re: Aerial for old valve sets.
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17th Jul 2019, 12:24 pm | #27 | |
Heptode
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Re: Aerial for old valve sets.
Quote:
Also an electrically short vertical is brilliant at picking up E field, i.e. local interference. So the only use for a vertical in the home is for Shortwave, VHF-FM, VHF Marine, Aeronautical VHF and DAB. |
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17th Jul 2019, 12:29 pm | #28 |
Heptode
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Re: Aerial for old valve sets.
The screened twin feed might have been microphone cable. TV or satellite (PF100) is superior and more defined to match. Or you can use 50 Ohm coax. Twin screened RF cable was last made for a specific IBM terminal and the only twin screened cable now is for audio, varying from 100 Ohms to 600 Ohms characteristic impedance depending on type.
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17th Jul 2019, 1:01 pm | #29 |
Dekatron
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Re: Aerial for old valve sets.
Maybe for long distance.
Interference aside: Quoted from the book ref. MW-LW.... …."The top of an inverted-L aerial, while quite unresponsive to the vertically-polarized surface wave from a transmitter, is affected by downward reflected radiation from the ionosphere, and this will increase the amount of signal-fading at medium distances (50-500 miles) during the hours of darkness. Vertical aerials are not subject to this phenomenon, and will in general be superior unless the height of the inverted L can be raised equal to that of the top of the vertical element...…...the aerial is non-directional with respect to the downlead, and it is quite immaterial which way the top section is run in relation to the direction of the transmitter" Lawrence. |
17th Jul 2019, 1:22 pm | #30 |
Heptode
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Re: Aerial for old valve sets.
Verticals are USELESS for LW / MW unless you can guy them about 20m tall, MINIMUM! Insulated base and guys. Even 20m is 1/10th wave at 200m (1.5MHZ) and 1/50th wave at 1000m (RTE is 252kHz = 1190m). So even a 20m mast is far better at picking up "near field" (local E field interference) than radio waves on MW & LW.
A vertical on MW & LW just picks up interference. A 20m wire at clothes line height is far better even than a 6m vertical on the chimney. Back when there WAS local daytime MW here. The car radio is dreadful on LW & MW compared to a 1929 TRF with a frame loop aerial in the lid. That works better than the chimney vertical or the 20m horizontal wire. The vertical is only any use for shortwaves, progressively better at higher frequencies. Frankly too the claim about medium distance fading is nothing to do with aerials. The book is very inaccurate. |
17th Jul 2019, 1:26 pm | #31 |
Dekatron
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Re: Aerial for old valve sets.
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17th Jul 2019, 1:31 pm | #32 |
Heptode
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Re: Aerial for old valve sets.
I've read other books and passed exams (Degree level not just the C&G Radio Amateur in 1972) and done aerial experiments for 50 years. That section is very inaccurate. It applies to SW, not LW & MW, and the fading is purely an ionospheric effect, mostly on shortwave and hardly affected by aerial type.
It's a book primarily for people servicing, not Communications Engineers or RF Design Engineers (I've done both those for a living). False Authority syndrome because it's good on other stuff. |
17th Jul 2019, 1:34 pm | #33 | |
Dekatron
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Re: Aerial for old valve sets.
Quote:
Lawrence. |
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17th Jul 2019, 1:36 pm | #34 |
Heptode
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Re: Aerial for old valve sets.
I have that book here as well as loads of other Professional texts, not service technician books.
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17th Jul 2019, 1:40 pm | #35 |
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Re: Aerial for old valve sets.
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17th Jul 2019, 1:41 pm | #36 |
Heptode
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Re: Aerial for old valve sets.
Because it's just waffle. I explained why in terms of aerial size and actual wavelength.
Read up on why local interference is mostly E field rather than M or Radio waves and why a wire picks that up well and a loop doesn't. A vertical would need to be on your chimney with earth radials and about 75m tall to be good for MW. That's equivalent to a 6m tall whip on shortwave or a 75cm whip on VHF band II. Edit: read ARRL and RSGB books on aerials. Last edited by Mike. Watterson; 17th Jul 2019 at 1:52 pm. Reason: Additional sources |
17th Jul 2019, 1:43 pm | #37 |
Heptode
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Re: Aerial for old valve sets.
My experiences indicate to me that at least for MW a larger frame aerial takes some beating. The main issue is that they can be unwieldy. My 80cm frame is not toatally rotatable where it is, but it works as well as the long wire the length of the loft > 10ft higher. A long wire in the garden may be better but wires are more susceptable to interference.
It's not something I have really looked into but fading does not seem to be affected by aerials in general, as they can only respond to the signal present. I do wonder though if certain types could be better responsive in responding to a wave in one plane which appears normally cancelled (ie faded) by a wave from a different plane meeting at the point of aerial? Or is it that I am showing lacking? |
17th Jul 2019, 1:43 pm | #38 |
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Re: Aerial for old valve sets.
In the UK I only recollect seeing vertical BC aerials on schools and industrial premises. For domestic use a longwire in the back garden or yard was the norm. They were ubiquitous, sometimes having corks threaded on them to act as a game guard where there was a pigeon loft nearby.
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17th Jul 2019, 2:02 pm | #39 | |
Heptode
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Re: Aerial for old valve sets.
Quote:
SMPSUs where the filter parts on supply needed to get CSA, EU/CE, FCC etc are later left out as there is no proactive retail sales sample testing. I have photos. DSL on overhead wires. So verticals that worked like car aerials do (with a base loading coil) on MW & LW in 1950s and 1960s are useless today. R4LW now only works in the car in the countryside. UK MW after dark now only works in the car in the countryside, even though even a TRF with a loop will work in the house. Though some cattle fencers have annoying Click Click Click. |
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17th Jul 2019, 2:16 pm | #40 | |
Dekatron
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Re: Aerial for old valve sets.
Quote:
Polarization...same with TV transmissions try horizontal in a vertical area...no good....that's the way it is so far as I know? Lawrence. |
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