My radio is a 38-690 that is located in my basement media room of my condo and is a walkout, approximately 50 miles north of downtown Detroit. Good reception is generally limited to the 2 local powerhouses (760 WJR and 950 WWJ, both 50,000 watt 24 hour stations), and my 1/4 watt transmitter using a cheap loop antenna. Night reception with a 50’ random wire inside allows some listenable stations up to 750 miles away, but with much noise. Tried the unused telephone wiring with very poor results.
I tried a 20’ random wire in the first floor attic (about 17’ above ground) using a 50’ coax lead in; no matter how I grounded the receiver, coax shield, or un-un the reception was worse and noise increased. I built a milk crate tuned loop with a twisted pair 10’ lead in and get about the same reception as the 50’ random wire....noise is reduced due to the directional/null that is inherent.
My questions are- what is the length of twisted pair lead in before noticeable degradation of performance? What should the twist per inch be for best performance? Would power supply conditioning help? Anyone have any other ideas of something that might work better? Outdoor antenna is not an option.
I have had luck using another 50' or so wire either perpendicular or parallel to your antenna and using that as your ground. With and w/o a real ground attached. A counterpoise. Will null out a lot of my static.
Easy to try
Bill
Quote:My questions are- what is the length of twisted pair lead in before noticeable degradation of performance? What should the twist per inch be for best performance? Any length of transmission line has loss...
For a twisted pair, #18 MTW, twist 1 turn in 2" should yield roughly 72ohms balanced. Warm the cable with a heat gun to "set" the insulation to reduce unraveling. Will want to use on a radio with a balanced input if practical.
That is the twist that I have, unfortunately the 38-690 does not have a balance antenna provision. I may try a Balun now that you have identified the twisted pair impedance.....a 9 to 1 should work I would think. Then I can play with grounding.
> unfortunately the 38-690 does not have a balance antenna provision.
Actually, it does. The 38-690, as well as the 38-116 Code 125, along with many other 1938 Philco sets, were designed for use with Philco's
High Efficiency Aerial, which was a form of doublet or dipole antenna.
The 38-116 Code 125 uses the same RF unit as the 38-690, and more information may be seen below in this snippet from the 38-116 Code 125 service bulletin.
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attachment=27118]
Ron,
Is there a diagrammatic of the Philco High Efficiency antenna online somewhere? I have looked but can only find pictures of the box.
Such antenna kits were popular with the shortwave radios of the 30's/40's Some used baluns, some did not, still others used a form of balanced matching at the antenna with taped or a combination of multiple wires, (web). This type of antenna is by design "aperiodic" meaning it is not specifically resonant at any point. The used of twisted pair brings balanced signals and some resistance to noise to the radio.
Most of these antenna designs can be replicated, those with baluns will be more difficult to replicate..
When I used to clean estates I would often find remnants of these antennas generally the twisted pair and occasionally the twin lightning arrestor and porcelain baluns.
Copying an Amateur antenna though would still work will have resonances in or near the amateur bands and often must be matched to the transmission line. Generally requiring a balun (that has loss) at each end of the transmission line.
YMMV
Chas
I did message the seller on behalf of the philco forum, and he has given us permission to use the pictures as we like. So at least we now know what it looks like in kit form.
I asked for a better price given the less-than-pristine condition but there are 'watchers' so he will wait and I don't blame him
-Simon (thinking about bidding)
Simon, CARPE DIEM ! You don't know when you will ever find one in original condition again. Of course, I advise to NEVER use it, but having an original will allow you to measure the various components, and figure out exactly what it consists of, so people can make approximate equivalents in the future.
Quote:I advise to NEVER use it, but having an original will allow you to measure the various components, and figure out exactly what it consists of, so people can make approximate equivalents in the future.
+1
Chas
I agree with keeping it as found, and to replicate the kit with today's available material.
We shall see....
Simon
Your radio. being in a condo, results in everyone's wall warts, pc/laptop power supplies (especially cheap ones), routers, wifi's and other devices ultimately destroying your reception. Interference is broadcast back through the condo wiring. Even if you disconnect all of your pc devices etc., others devices in the condo will destroy reception. Use a battery powered am radio to walk around your condo unit to hear the interference (same interference) and where it is less, you might find a better location for the radio. Then you can attack a better antenna solution for the radio.
Ok Mikethedruid, I CARPE DIEM'd this thing, and am now waiting on delivery.
This will be interesting, especially the 'transformer' with ground wire going to a 'standpipe'
I will put the white gloves on and not break it. I just hope the packing and shipping is kind.
-Simon
Quote:especially the 'transformer' with ground wire going to a 'standpipe'
Coax was not in wide use at the time of the creation of this antenna kit. So twisted pair was the receiving transmission line of the day. If the wire size and insulation thickness is like what was common power cord of the era, the nominal impedance would be 72 ohms. This is an ideal match for a dipole, however, such a dipole is good only at resonance and not so good off resonance. Off resonance the feed-point impedance rises. Twisted pair is tolerable to this off resonance much more than coax. The twisted nature cancels external noises and metal objects in near proximity but should not be against such conductors like metal siding or stucco lath.
The antenna input device should be very interesting to see how broadband matching is used or even if matching need be accomplished, may be no more than a lightning protection device and a terminus for conductors to enter the building.
RCA also provided antenna kits, just as hard to find in new condition.
Thanks for providing the antenna data...
Chas