Dual-Band X-Frame Shortwave RX Loop

Having a few nice vintage shortwave radios around, I decided it would be nice to actually USE the dang things once in a while 🙂   And given the almost unusably noisy performance of indoor short wire antennas, I made some loop antennas to cover approx 3 – 18 MHz to see if performance could be enhanced.

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Hallicrafters S-53 Ressurection

I really like the classics, the glow of tubes and smell 🙂 of transformers And having a short window of free time decided to pick up a poor ugly semi-rusted Hallicrafters S-53 for cheap. I was interested in it’s somewhat unusual 2.075 MHz IF frequency (used to enhance image rejection). I decided to try out a weird idea in regard to not really restoring, but rather quickly resurrecting an old not-so-valuable boat anchor.

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Cerwin Vega AT-15 Xover Schematic

I visually traced out the circuit layed out on the circuit board and then measured with Fluke BK Precision model 886 LCR meter.

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ANL Add-On for Hallicrafters SX-16

Out here in beautiful rural East Texas, the noise levels on the HF bands are not too awful bad – IF you have a decent noise limiter or blanker.

We have some nasty short spike shortwave noise at a 33 mSec rep rate. It may be electric fences? our cable system?, but I have not tracked it down yet since I did not even know it existed due to modern receiver’s good noise blankers. (This explains the picket-fence spectrum of certain frequency ranges on the IF waterfall display).

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Small RX Loop Analysis – An Excel Sheet – Fields, Voltages, Noise Levels…

First  cut at an Excel spread sheet to analyze a small RX loop’s noise and frequency response performance.


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Why We Don’t Like Unintended Common-Mode Current Flow On Our Transmission Lines

You may have wondered why some of us keep yammering on regarding “Common Mode Current” on the coax, and why common-mode prone designs (like end-feds and JPoles) are so problematic.  Here’s an example of a 915 MHz antenna for a wideband comm system prototype I am designing. I decided on a wide-band vertical dipole.

In the pix below, we see the dipole front, and the dipole rear in it’s final form (sans heat shrink) – with a single loop binocular type 61 core performing the 1:1 current BalUn / choke function (they are the same thing):

Dipole_Front

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Solar Eclipse Data Showing Propagation Enhancement on the Lower HF Bands

Solar eclipse data shows interesting enhancement of my signals on 160 M (1.8 MHz) during the event. Similar data for another station is shown for 80 M band (3.5 MHz) also.

Eclipse WSPR Data

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“Z-Substitution” Method for Measuring UNUN Loss Into Real-World Load Impedances

Often one sees characterization and loss measurement of UNUNs using an SWR measurement of the UNUN when terminated in it’s transformation resistance, or with the “back-to-back” 2-x-Loss method.  Each method reveals something about some quality of the UNUN, but neither method tell us much about how much power the UNUN will deliver to the actual antenna.
unun-2In this article I will show a simple method of measuring actual UNUN losses into the actual load impedances that the UNUN will see in service.
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Classic W8JI Limited Space Antennas Article

Excellent technical article regarding limited space antennas:

Cap Hat Dipole

Limited Space Antennas

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We Don’t Need No Stinkin’ Sweep Generator …

… or any signal generator for that matter in order to align the IF section, including the notorious crystal filter and phasing circuits, of Vintage Receivers like the Hammarlund HQ145 and HQ129X.  It takes about a half hour once you get used to it, perhaps an hour or so the first time.  I’ve aligned my Hammarlunds with this procedure and found the units to be very close to factory spec as measured with standard lab equipment.

A layout diagram, such as the one for the HQ145 below, is useful ===>

hq145toplayout-1
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