Topband: Loopstick for 160m

[Walter E. Miller, AJ6T]

mmm
"Ted Roycraft" <roycraft@research.att.com> wrote:

> In the "ARRL Antenna Compendium, Vol. 6", there is an article describing
> a tuned, ferrite rod, magnetic receiving loop antenna - "The Optima
> 160/80-Meter Receive Antenna" by Richard Marris, G2BZQ.  I'm tempted to
> build this thing to see what it does to my receive noise level on 160.
> The only reason I'm reluctant to go ahead is that the price of the
> required ferrite rods alone is over $120.  (It uses a total of 72 inches
> of 1/2 inch diameter Amidon type 61 ferrite rod).
>
> By the way, I'm using an inverted-L for both transmit and receive at the
> moment.
> I'm located on a suburban lot so anything like a Beverage antenna is out
> of the question but I'm still looking to improve my received SNR.
>
> Does anyone have any comments on this loop antenna or one like it?
>

Ted,

I was also intrigued by the loopstick antenna described in the ARRL
Antenna Compendium, Vol. 6.  Because of the cost of the ferrite rods, I
chose to construct a smaller version made with only four 7.5 x .5 inch
rods purchased from Amidon (PN R61-050-750, $15 per rod).  I tried to
find some cheap rods from discarded AM broadcast radios at a local flea
market without much success.  The rods were stacked and held together
with electrical tape (overall length 7.5 inches). I built my prototype
on a wooden base, with the rods encased in a wrap of aluminum foil.  The
incomplete foil shield is open on the ends and has a slit along its
entire length to avoid the "shorted turn" effect.  An alligator clip and
jumper wire join the foil to the circuit common point.  The L1, L2 and
L3 windings were per the article, but the gap between L1 and L2 was
somewhat less than the 6 inches shown in the original design.  Two
junkbox air variable caps rounded out the prototype.

To my surprise and delight, the loopstick resonated on 160m without any
further modifications.  I tuned it up using an MFJ259 SWR analyzer, and
I was able to get the SWR down to 1.1 at 1830 KHz.  Careful adjustment
of both caps is necessary to match the antenna.  The input cap (C2) has
a big effect on the SWR, but it doesn't really change the signal pickup
much.  The main cap (C1) determines the resonant frequency, and the
noise rush in the receiver is quite obvious as C1 is tuned through
resonance.  The 2:1 SWR bandwidth is very narrow, just a few tens of
KHz.

In my tests the loopstick was mounted about four feet above ground.  The
sensitivity of the loopstick is way down compared to the 55-foot
vertical "T" antenna that I use for transmit.  Lacking a calibrated
S-meter, I estimate it is down at least 30 dB.  I use an ICE
preamp/filter with the loopstick to bring the signal level back up.  The
antenna is very good at nulling out local QRN and QRM (especially
obvious on local computer birdies), but it is not very directional on
skywave signals.  The S/N ratio is better on most received signals, and
sometimes the loopstick has made the difference between Q5 copy and no
copy, but the effect is usually not dramatic.  It did help me work K5K
from California (but later they were quite loud on the vertical), and I
can copy JAs on it, but I have not heard any really long haul DX yet
with this antenna.

I rate this loopstick as somewhat helpful, about on par with other small
receiving antennas I have experimented with (300 foot Beverages, larger
loops, low wires, etc.).

73, Walt
AJ6T  Saratoga, CA


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