RadioBanter - View Single Post - Putting a Ferrite Rod at the Far-End of a Random Wire Antenna ?

November 14th 04, 08:30 PM

On 14 Nov 2004 06:42:51 -0800, (RHF) wrote:
So my 'focus' is on a "Low Noise' Inverted "L" Antenna that is
100 Feet long. Placing the Ferrite Rod at the "Top" of the
Vertical Leg where the Antenna Wire Element transitions from
Vertical to Horizontal. Rig the Antenna Wire Element with 33
Feet Up to the Ferrite Rod AM/MW Ferrite Coil and then have 67
Feet Out to the Far-End. Would this possibly work any better
then having the Ferrite Rod and Coil at the Far-End ? and Why ?

Hi OM,

If you want to resonate it at ONE frequency (the supposed reason for
having that lump at the end) then you would need an inductive
reactance of
1000 Ohms at the feedpoint to render the antenna a resistive 6 Ohms
and a poor average gain of -8.7dB;
or
1000 Ohms at 3' up to render the antenna a resistive 7 Ohms and a poor
average gain of -8.5dB;
or
1300 Ohms at 16' up - 7.8 Ohm Antenna @ -8.7dB;
or
1400 Ohms at the top - 9 Ohm Antenna @ -9dB;
or
1900 Ohms 16' out the horizontal leg - 11 Ohm Antenna @ -9.6dB;
or
2600 Ohms 32' out - 12.5 Ohm Antenna @ -10 dB;
or
5800 Ohms 48' out - 14.6 Ohm Antenna @ -10.5dB;
or
10200 Ohms 58' out - 15.4 Ohm Antenna @ -10.6dB;
or
56500 Ohms 64' out - 15.2 Ohm Antenna @ -10.6dB.

You should note the progression of increased inductance required to
tune this antenna to 1MHz (ymmv). At any other frequency and bingo,
all bets are off. You should also note there is only a 2dB spread
(AVC would quickly put that to rest). Also, the numbers above presume
you performed the chore of laying out 16 X 33' radials.

Let's compare that to the advertised antenna
9200 Ohms at the bottom - 32.5 Ohm Antenna @ -26.8dB;
or
13700 Ohms at 8' up - 35 Ohm Antenna @ -22dB;
or
108750 Ohms at 15' up - 39.6 Ohm Antenna @ -19.9dB.

This presumes the single lead-in laying along the ground (where most
of this Antenna Ohmage resides.

This, further, would put your antenna roughly 10dB ahead (sans
loading, and using a simple tuner) of the $95 spread. I hope you
notice the parenthetic statement of a tuner - it does the same thing
as these loads, and you get to tune to more than one frequency.

I will leave it up to you to determine how many wraps around how many
rods would give you 110000 Ohms inductive reactance at 1MHz.

In my 'imagination' for the AM/MW Broadcast Band; the Vertical
Up-Leg would act like an Omni-Directional "Tuned" Vertical Antenna
(since the Ferrite Rod would be Vertical); with the Horizontal
Out-Arm of the Antenna acting like a Top Hat for the Vertical.
Then at the Shortwave Frequencies the 'effects' of the Ferrite
Rod would become less and the Antenna would simply act as a
normal SWL'ers Inverted "L" Antenna.

Well, let's just test that hypothesis with the load at the top.

What you get is fairly well tuned at 10MHz (SWR 1.7) with omni
coverage in both horizontal and vertical. For other bands, of course,
a tuner would be useful.

Now here is another thought; would having the Ferrite Rod with the
Antenna Wire wrapped around it at the Far-End of the Horizontal
Antenna Element cause the AM/MW Broadcast Band reception to be
Directional 'Off-the-Sides' of the Inverted "L" Antenna ? ? ?

NO. For short antennas and low horizontals, there is almost no
directionality. What you are attempting to do is to capture the
quality of small loops (which are what the ferrites are) of being
directional. The signal from the parasitic wire (what is not part of
the small ferrite) is going to fill in the nulls and deluge its poorer
sensitivity as well. You can of course test this answer by finding a
way to answer my question inferred above about how many wraps and how
many rods to achieve 110000 Ohms inductive reactance.

73's
Richard Clark, KB7QHC