AM DX ... from Your Car
Auto Radios have pretty good AM capabilities. I have converted several of them for use in the shack by
adding an external speaker and a simple 12 volt power supply. I usually feed them with some form of
outdoor wire antenna. These radios are made with very selective and sensitive front ends. The reason
for this is because the radio has to receive signals in the frequency range of from about 1/2MHz to
1.5MHz (.53 MHz to 1.7 MHz). But imagine that it has to do a decent job of receiving those frequencies
from an antenna that is nominally tuned for 100 MHz! Yup, that's right. Most car antennas are about 30 to
33 inches long - tuned for the middle of the FM broadcast band - with a resonant frequency of 90 to 98
MHz - about a 100:1 mismatch for AM broadcast! [100/1=100]
So there is no doubt that these little radio wonders are truly great performers.
I spend a lot of time listening to AM DX from my shack, but some time ago decided I wanted to do that in
my car. And why not! The radios are digitally tuned, have tight front ends, good IF filtering, and nice,
pleasant audio bandwidth.
Here's what I did for my traveling (and listening) pleasure.
The antenna element is a 36 inch long brass 1/8 inch brazing rod.
That's about 6 inches longer than the stock auto antenna. Although I
could have used something larger, I chose a 30 meter Hustler loading
coil to base-load the element (an 80 and 40 meter coil actually caused
overloading on local stations!) Measurements with my antenna
analyzer indicate that the antenna system's resonance has gone from
about 100MHz down to 10MHz. A mismatch of only 10:1 - that's a 10:1
improvement over the original 100:1 and the reception verifies that - it's
truly amazing. By-the-way, FM reception is as good as always. NOTE:
The old original car antenna is only extended for comparison purposes
- normally, it's pushed all the way down.
Look at the detail photos. You can see that I prefer "body
mount" brackets rather than "lip" mount. I think Diamond
or Comet sell these. They don't mar the paint with
clamp-on hardware and stay stationary when you open
the trunk lid. They are very easy to mount, down in the
rain-drain channel inside the fender. Drill 3 small holes
and bolt the thing on, nice and solid, to the car's body ...
not the trunk lid.
I used a PL-259-to-3/8" adaptor for the coax cable and a
scanner/CB-type stainless spring for strain relief. Again,
the gray loading coil is an old Hustler that is designed to
handle more than 100 watts of transmit power - so it has
a pretty high Q-factor (XL / R), but I'm sure any brand will
Also note, for general safety reasons, I threaded an
aluminum spacer on the top end - no sharp pointy ends.
The next rainy weekend, dig around in the junk boxes
and see what you can come up with - maybe look for old
CB hardware at yard sales. Use any kind of mount you
want, any element material (stainless would be cool), a
spring or not, and loading coil of unknown origin - maybe
wind your own. Try it. Experiment a bit. If you get "local
overload", then back off on your design, or add an RF
gain control somewhere (I suggest a 5K pot).I'll bet you
there ain't too many people on coastal Connecticut who
can take the wife shopping whilst listening to WLW in
Cincinatti, Ohio, KDKA in Pittsburg, Pennsylvania, WLS in
Chicago, and WOWO FT. Wayne, Indiana.
I usually leave the radio club so I can be home by about 10:30 - need a little snack before bed. Each push of the
tuning button advances the radio 10KHz (one AM broadcast channel). During that 20 minute drive, late at night, I
can usually hear a station on every channel - some are weak and noisy and an ID can't be made, but that's very
few, and besides, every night is different. Clear, cold winter nights seem best, unless a storm front is approaching.
Hot, sticky summer nights are the worst, especially with lightning crashes off in the distance. Spring and autum are
hit-and-miss. Well you can't have everything. I guess I should count my blessings: One wife - four seasons.
Experiment with this project - TRY IT - HAVE FUN.
|For home brewers, refer to the photo
Working up from the bottom, the parts are:
1. Channel (groove) mount.
2. SO-239 to 3/8" stud adaptor (with nylon
3. 30 meter (10 MHz) loading coil Hustler
4. CB-type spring.
5. 1/4' x 20 stud to 1/8" rod adaptor.
6. 3' x 1/8" brass or bronze brazing rod.
7. Inside the trunk is a PL-259 to Motorola
coax adaptor (Male or Female, as req'd.).
All the components are available from
various mobile radio supply houses. Just
do a search on "mobile radio antenna,"
mobile ham antenna" or "CB antenna."
Even the actual element (whip or stinger)
can be found in stainless steel or chrome
plated brass. Other keywords are Hustler,
Comet, Diamond, Firestick. Also look at
Radioware and Radio Works for connector
adaptors, plus Universal radio and Grove
Good luck. Take your time and enjoy the
project -- remember to report back -- we
want to know.