Noteworthy Circuit: Low-Noise Broadcast Antenna
March 1969 Radio-Electronics

March 1969 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Having only been seriously (somewhat) on the amateur radio scene since earning my Ham license in 2010, I previously did not pay a lot of attention to antenna construction and interconnection to radio gear. My experience was limited to routing 300 Ω twin-lead transmission line from the rooftop TV antenna down to the indoor set. I also made a lame attempt at stringing a long AM antenna wire in an attempt to pull in distant stations at night. It worked better than the built-in coil on the ferrite rod, but was nothing to brag about. Evidently, there was a better method than a single lead for an AM antenna. This 1969 Radio-Electronics magazine article suggests using a balun to create a balanced twin line from the wire antenna to the receiver, then a balun back to single line at the receiver end. Probably not many people are using such configurations anymore, but the subject was considered worthy of a page in a national publication in the day. Maybe there are still people around who can use such advice.

Noteworthy Circuit: Low-Noise Broadcast Antenna

A good outdoor antenna is essential to satisfactory AM broadcasting-band DX'ing. For most purposes, it may be a single-wire flat-top, as long and as high as practical. Man-made electrical interference is one of the major obstacles faced by the serious DX'er. Most of this comes from high-tension power lines, automobiles and industrial, commercial and domestic appliances. It is strongest close to the source and tapers off rapidly until it may not be objectionable a few hundred feet away.

For noise-free reception, the first step is to get the antenna as far as possible from noise sources. The second is to replace the most-often-used single-wire lead-in with a low-impedance balanced transmission line matched to the antenna and receiver by transformers. A low-noise broadcast antenna like this is shown in the diagrams, taken from Electronics Australia.

The lead-in is plastic-covered twisted-pair coupled to the antenna by T1 and to the receiver by T2. Both transformers are wound with No. 26 enameled wire on 2" diameter forms. For T1, wind the primary with 100 turns and mark the starting end P1 and the terminating end P2. Wrap with a single layer of insulating tape. Add an electrostatic shield made from thin brass 1" wide and just long enough to leave a 1/8" gap when wrapped around the center of the coil. Connect the shield to P2 and add another layer of tape. The secondary is 25 turns wrapped around the center of the shield. The starting end is S1, the other is S2.

Wind T2 with the 25-turn primary next to the form. Add the tape, electrostatic shield and then wind the 100-turn secondary. Tap the secondary at every tenth turn from the fiftieth on.

Transformer T1 should be water-proofed or saturated with coil dope and then mounted in a shield can at least twice the coil diameter. Mount it on the mast close to the antenna. Connect the end of the antenna to P1 and run a wire to ground from P2. Do not ground the shield can.

Mount T2 close to the receiver and connect P2 and P1 to the ANTENNA and GROUND terminals, respectively. Connect S1 and S2 to the lead-in. Tune in a weak station and try all the P1 taps to find the one giving the greatest deflection on a tuning indicator or a vtvm connected to the avc line. R-E

Posted April 19, 2023