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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 16th Jun 2020, 11:16 pm   #21
William78
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Default Re: Ferrite Rods for MW/LW - Design Compromise?

I have just remembered that my Ferranti 255T has two ferrite rods, one for MW and one for LW. Adjustment is done by moving a ring along the rods (under the red paint in the photo below).

William
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Old 16th Jun 2020, 11:21 pm   #22
JohnBG8JMB
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Default Re: Ferrite Rods for MW/LW - Design Compromise?

mike
Ferrites at vhf

In 1972-3 I was deeply involved in vhf paging rx design for Multitone. At 153 MHz, we used a F29 rod [neosid] 10mm dia x 40 or 50 long. Optimum results were obtained with a coil wound with copper foil -AFAIR, 10mm wide and 1.5 or 2..5 turns; it was necessary to enclose as much of the rod as possible in the coil. This was for the first area wide pager for the GPO
We used something similar for firemen's and lifeboat pagers on ~80MHZ

Neosid still list F29 but I've no idea how available it is.

John G8JMB
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Old 17th Jun 2020, 12:15 am   #23
Boulevardier
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Default Re: Ferrite Rods for MW/LW - Design Compromise?

Quote:
Originally Posted by JohnBG8JMB View Post
mike
Ferrites at vhf

In 1972-3 I was deeply involved in vhf paging rx design for Multitone. At 153 MHz, we used a F29 rod [neosid] 10mm dia x 40 or 50 long. Optimum results were obtained with a coil wound with copper foil -AFAIR, 10mm wide and 1.5 or 2..5 turns; it was necessary to enclose as much of the rod as possible in the coil. This was for the first area wide pager for the GPO
We used something similar for firemen's and lifeboat pagers on ~80MHZ

Neosid still list F29 but I've no idea how available it is.

John G8JMB
Thanks John - interesting. I was mainly curious about Band II - I have never seen a domestic VHF receiver that uses one.

Mike

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Old 17th Jun 2020, 12:58 am   #24
Jolly 7
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Default Re: Ferrite Rods for MW/LW - Design Compromise?

Of late I've been experimenting with various types of MW antennas, both ferrite and loop. By adjusting the coil position(s) on ferrite I've been comparing the performance of different loopstick- tuning capacitor combinations in homebrew TRFs to get the maximum possible coverage of the MW band. My observations are that some tuning capacitors work better than others with the same coil in that they provide more coverage of stations. The polyvaricon ones seem to outperform the older metal ones, although this could be due to hand capacitance or other factors ?
My experiments with external frame loop antennas, typically sold with some stereo systems around two decades ago, are also yielding some good results. I am easily able to pull in weaker stations with a Sony frame loop e.g. Radio Caroline on 648 kHz and Spirit Radio on 549 kHz. Its inductance is 0.02mH as measured on my meter. Picture attached.
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Old 17th Jun 2020, 2:46 pm   #25
G6Tanuki
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Default Re: Ferrite Rods for MW/LW - Design Compromise?

I remember seeing a small MW/LW transistor-radio from the early-sixties where there was only a MW winding on the ferrite rod, and for LW a separate coil-in-a-can located on the PCB was added in series.

I've always been fascinated by ferrite rods for MW/LW and SW (particularly SW direction-finding) - there does seem to be quite a degree of variability in terms of design - some using a short, fat semi-pile-wound Litz coil and others using a longer coil of thicker single-strand wire but wound with an inter-turn-gap of a wire's diameter or more.

For a MW/LW radio I've always thought self-capacity between the windings must be a significant issue - particularly if you want to avoid the tuning-scale becoming horribly cramped at the HF end of MW (where the 'interesting' stations were in the 'sixties) - sticking a small capacitor in series with the tuning-gang for use when the '208' button was pressed always seemed a bit of a kludge).
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Old 17th Jun 2020, 4:11 pm   #26
kalee20
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Default Re: Ferrite Rods for MW/LW - Design Compromise?

Self-capacitance is one reason for using silk/cotton/rayon covered wire - it keeps the turns of copper farther apart.

Another method is to just use enamelled wire, but wind bifilar with a spacing wire of suitable diameter, which is then stripped off. Enamelled wire is much cheaper, but labour costs of doing this are obviously higher.

William - thanks for the photo of the Ferranti! It's good to know that twin rods DID occasionally get used in commercial manufacturer.
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Old 19th Jun 2020, 8:00 pm   #27
WaveyDipole
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Default Re: Ferrite Rods for MW/LW - Design Compromise?

I am curious - are the two coils adjacent to each other on the rod, or are they at opposite ends as seen in some radios? Does it make a measurable difference to the effect?
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Old 19th Jun 2020, 8:24 pm   #28
Restoration73
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Default Re: Ferrite Rods for MW/LW - Design Compromise?

In the earlier era of transistor portables, it was almost mandatory to offer a connection
for external i.e. car aerial. For the majority of models this used an additional coupling
winding mounted between LW and MW coils. Some portables and tuner amplifiers
used an extra winding for low HF coverage, typically 2-5MHz, whereas a few had
a separate rod. The more expensive portable radios used separate inductors switched
in when "Car" was selected, and therefore perform like car-only receivers.
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Old 19th Jun 2020, 8:33 pm   #29
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Default Re: Ferrite Rods for MW/LW - Design Compromise?

Back in the day, if you fitted a car radio, you had to have a licence for it, quite separate to any licence for radios in your home. BUT if the radio was a portable to the extent that it could be taken out of the car and used with its own battery and antenna, then it was officailly a portable radio and covered under your home radio licence.

It mattered to the tune of a few quid per annum, and it shaped the design of radios.

Something similar applied to tellies, so you got portable TVs with internal batteries as products for a while.

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Old 19th Jun 2020, 9:22 pm   #30
kalee20
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Default Re: Ferrite Rods for MW/LW - Design Compromise?

Quote:
Originally Posted by WaveyDipole View Post
I am curious - are the two coils adjacent to each other on the rod, or are they at opposite ends as seen in some radios? Does it make a measurable difference to the effect?
That's one of the things I tried.

The MW and LW close to each other on a single rod, is bad news. Keeping separate minimises the interaction (whether LW coil is O/C or S/C on MW), but it hurts sensitivity because for best pickup, the coil should be towards the centre of the rod (See Radio Wrangler's post).

And if they are both towards the centre, then they'll be close together.
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