Attempting my first successful broadband wire loop antenna project
- Hello, group.
I have not written to you for such a long time simply because I have
been working on a simple broadband loop antenna project. In addtion
to the antenna, I have also attempted my latest successful
regenerative radio receiver project to which I can attach this
Basically, the broadband wire loop was made out of a 25-foot length
of #14 AWG solid copper wire, THHN insulated, fashioned into about
six feet each leg, an impedance matching transformer with a ratio of
32:1 (1600 ohms primary, 50 ohms secondary), and a coaxial cable used
for a connection to the radio via a panel mounted TV coaxial
connector. The transformer consisted of 37 turns of #28 AWG enamelled
copper wire on an FT-140-77 ferrite toroid for the primary and 5
turns of #24 AWG enamelled copper wire for the secondary winding
opposite from the primary winding. The ends of the primary winding
were connected to the ends of the wire loop , and the ends of the
secondary winding were connected to center tip of the connector and
its ground lug, which is attached to the connector's threaded shell.
An RG-59U cable is used as the feed, or lead-in to the receiver.
Once the antenna was assembled, I hooked it up to a regenerative
radio receiver, tuned in Radio Disney 1590, and sure enough, the
antenna works! Its reception pattern is bidirectional (figure-8) with
deep nulls for MW. The antenna may even cover shortwave as well as
I am happy that my wire loop antenna works!
- At 16:12 07/04/24, Digest Number 800 wrote:
>1. Attempting my first successful broadband wire loop antenna projectWhy did you choose such a high input impedance as 1600 ohms?
> Posted by: "cwilliams1002002" cwilliams1002002@... cwilliams1002002
> Date: Mon Apr 23, 2007 2:18 pm ((PDT))
>[snip] the broadband wire loop was made out of a 25-foot length
>of #14 AWG solid copper wire, THHN insulated, fashioned into about
>six feet each leg, an impedance matching transformer with a ratio of
>32:1 (1600 ohms primary, 50 ohms secondary)
I have a similar sized loop (approx. 30 feet) connected direct to my
receiver and I find it works better connected to the 50 ohm input
than to the 600 ohm input. I'm not tempted to a higher impedance.
(For your copper-tube loop, described last September, you used a
1:9 step-UP turns ratio, or 1:81 impedance ratio, apparently into
a 50 ohm input preamp. The loop lengths are not drastically different,
so why such an extreme reversal in transformer ratio?)
Since the radiation resistance of a "small" loop is tiny, its resistance
and inductance are small, and as it isn't resonated [**], I cannot see
a purpose in such a large "matching" impedance.
[** Using RJELOOP3, its inductance is 10.2uH. A nomogram gives
its reactance as 1600 ohms around 30MHz - but you won't get
there, because its self-resonant frequency is about 20.5MHz.
At 1590KHz, Rr=0.28 miili-ohms, Rloss=0.49 ohms and ZL=101 ohms.
BTW, 986pF would tune it, giving a Q of 209 and an impedance of 21.2K
at resonance and a sensitivity about -32db wrt a 1/4 wave vertical.]
>[snip] The transformer consisted of 37 turns of #28 AWG enamelledIs this the same impedance matching transformer referred to above?
>copper wire on an FT-140-77 ferrite toroid for the primary and 5
>turns of #24 AWG enamelled copper wire for the secondary winding
>opposite from the primary winding.
The turns ratio is 37:5 or 7.4:1, so the impedance ratio is 54.76:1,
which is quite a lot different to 32:1 and would result in an input
impedance of 2738 ohms rather than your nominal 1600.
To achieve your stated 32:1 impedance ratio the turns ratio would
need to be approx. 5.66:1, or close to 28:5.
Also, a ferrite has an optimum frequency range, above which it
presents a resistive rather than reactive impedance making it
unsuitable for inductors and transformers but excellent for RFI
suppression. Type 77 is recommended for transformers from
1 KHz - 2 MHz, so you might find it limiting for shortwave,
especially as you choose to put the windings on opposite sides
of the toroid and thus rely entirely on core flux for your coupling.
Instead consider type 43 (.01-10 MHz) or type 61 (.2-100 MHz).
There isn't a lower frequency limit as such, but the lower
permeability of high-frequency ferrites demands more turns
(perhaps impractically more turns) making a high permeability
core more attractive where its frequency range permits.
>[snip] tuned in Radio Disney 1590 .. the antenna works!To give us an idea of how impressive this feat is, please tell us
how powerful the Radio Disney Tx is and how far from your QTH.
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