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Pre NARBA Questions For Senior DXers

S

Schroedingers Cat

Are there any DXers here who DXed and listened to AM before NARBA took effect in 1941?

What was the typical groundwave reception (day and night) range of 100 and 250 watt Class IVs? Was there any regularly received DX on those stations? Same question for typical 1000 watt Class IIIs and other classes.

I'm sure we could model the coverage if we had the software. Perhaps R. Fry knows the radiation efficiency of the T shaped hammock antennas commonly used before NARBA.

Anecdotal recollections of the groundwave range of particular stations and commonly heard skywave stations in areas where you lived or DXed are welcome.
 
Schroedingers Cat said:
Perhaps R. Fry knows the radiation efficiency of the T shaped hammock antennas commonly used before NARBA.
Click to expand...

Responding to SC's request, NEC-2D studies were done for a base-fed, MW tee antenna. The vertical conductor was 50 ft high, and connected to the center of a horizontal conductor 200 feet in length, using non-conducting vertical supports at the ends of the horizontal conductor. The OD of both conductors was 1/4 inch. The NEC model had no geometry or segmentation errors. The frequency was 1400 kHz. Ground conductivity = perfect, to determine the "efficiency" or IDF of the antenna system.

The radiation pattern is vertically polarized, has maximum gain in the horizontal plane, and a null at the zenith. The h-plane radiation pattern is circular.

The resistance of the connection to the ground plane was first set to 2 ohms to simulate a typical set of 120 x 1/4-wave buried radials, and then to 10 ohms to simulate a set of fewer and shorter buried radials that might have been used in the early days of AM broadcasting.

Results for an applied power of 1000 watts --

  • 2 ohm Connection: Feedpoint Z = 9.7 +j 15.2 ohms, IDF at 1 km = 265 mV/m
  • 10 ohm Connection: Feedpoint Z = 17.7 +j 15.2 ohms, IDF at 1 km = 197 mV/m

Of interest here is that if this tee antenna had a zero ohm connection to the perfect ground plane, its IDF at 1 km for 1 kW of applied power would be 298 mV/m.

RF
 
Thanks for the information, R. Fry. One thing that is great is that the laws of Physics don't change, at least observably during the history of radio. As I suspected, the efficiency is substantially less than a quarter wave monopole with radials. I'll have to dig up a computer that has the software on it to plot out some coverage maps. Most Class IVs were 250 watts or 100 watts back in that time, and the vast majority of smaller market stations were of this variety. 1200, 1210, 1310, 1370, 1420, and 1500 were the Local Frequencies. A regional with even 1000 or 500 watts night had substantial skywave DX potential. My late father sometimes played as a live musician on WREO Lansing, MI back in the 1920s and 1930s, and he said they got letters from as far away as Montana that heard the program. Radio history sites show WREO as 500 watts on 1050 and 1300 at various points in its history. There is a post card showing the WREO antenna, which is a T shaped hammock antenna between two short towers. It was apparently not on the Olds/Boji Tower which has had at least two FM antenna installations in the last 50 years.

I still invite people with anecdotal stories like these to post about day and night service areas of lower power stations from pre 1941.
 
Schroedingers Cat said:
... As I suspected, the efficiency is substantially less than a quarter wave monopole with radials.
Click to expand...

Yes, because even though the r-f current distribution along the vertical conductor of this tee antenna system essentially is uniform (unlike that of a 1/4-wave, unloaded monopole), the radiation resistance of this tee system is only about 21 % that of the monopole.
 
Schroedingers Cat said:
I'll have to dig up a computer that has the software on it to plot out some coverage maps.
Click to expand...

Here is set of numbers for that tee antenna for 1400 kHz, 250 watts applied power, earth conductivity = 5 mS/m, d.c. = 13

km Field Intensity

(to follow)
 
The National Radio Club has started to scan most of the back issues of their "DX News" going back to the 1930s, and they have them posted at www.e-dxn.com. I don't know if the back issues are members-only or not, but if they are you can get a membership for $15, and it's well worth it to go through nearly 80 years of DX reports.

It's amazing how well the graveyarders got out back then. It helped that most were not on 24-hour schedules, so the band got quieter at night, and especially on Monday mornings when most stations shut down by midnight for maintenance. It's not uncommon to see coast-to-coast receptions of the handful of early graveyarders (KGFJ 1200/1230 Los Angeles, for instance) that were running 24/7 or nearly so.
 
Schroedingers Cat said:
I'll have to dig up a computer that has the software on it to plot out some coverage maps.
Click to expand...

Here is set of numbers for that tee antenna for 1400 kHz, 10 ohm r-f ground connection, 250 watts applied power, earth conductivity = 5 mS/m @ d.c. = 13

meters Field Intensity, mV/m
1000 53.8
2000 23.6
3000 13.9
4000 9.4
5000 6.8
6000 5.1
7000 4.0
8000 3.2
9000 2.6
10 km 1.09
15 0.49
20 0.25
25 0.16
30 0.10

RF
 
Ah, the pressures of the R-I edit clock. Unfortunately the values in my last post for 9 km and below were based on 1 kW applied power, rather than 250 watts.

Here are the correct values for a set of numbers for that tee antenna for 1400 kHz, 10 ohm r-f ground connection, 250 watts applied power, earth conductivity = 5 mS/m @ d.c. = 13

km Field Intensity, mV/m
1 26.9
2 11.8
3 7.0
4 4.7
5 3.4
6 2.6
7 2.0
8 1.6
9 1.3
10 1.09
15 0.49
20 0.25
25 0.16
30 0.10

RF
 
Thanks for your input, Scott, and your extensive calcuations, R. Fry.

I found some information about WREO online, and I emailed my additional story at the online request of the website owner. The email came back, and sadly, I discovered through a search for a new email that he had passed away.

The transmitter location is in the website regarding WREO. It was located at the REO Clubhouse on Washington Ave. near Baker St. in Lansing.
 
In re-checking my NEC calculations posted earlier in this thread, it was found that they didn't agree well at all with the values shown for this radiator efficiency in the appropriate FCC groundwave propagation chart. Looking back at my NEC model I found and corrected the reason for the problem. My apologies.

Following is a short table with the distances to the field intensity values shown, for that tee antenna on 1400 kHz with 250 watts applied power, a ten ohm r-f ground connection, and an earth conductivity of 5 mS/m, d.c. 13.

Distance, FCC Field NEC-2D Field
miles mV/m mV/m

2.7 10 11.2
4.2 5 5.6
6.3 2.5 2.8
10 1 1.1
13.9 0.5 0.53
28.3 0.1 0.106

//
 
Scott Fybush said:
The National Radio Club has started to scan most of the back issues of their "DX News" going back to the 1930s, and they have them posted at www.e-dxn.com. I don't know if the back issues are members-only or not, but if they are you can get a membership for $15, and it's well worth it to go through nearly 80 years of DX reports.
Click to expand...

It is members only, but as you say, it is worth $15 to subscribe.

It's amazing how well the graveyarders got out back then. It helped that most were not on 24-hour schedules, so the band got quieter at night, and especially on Monday mornings when most stations shut down by midnight for maintenance. It's not uncommon to see coast-to-coast receptions of the handful of early graveyarders (KGFJ 1200/1230 Los Angeles, for instance) that were running 24/7 or nearly so.
Click to expand...

Even in 1960, I managed KTIP in Porterville, CA, on Sunday morning with most of the co-channels sleeping. Of course, at that time the power maximum at night was 250 watts. And there were several 100 watters left, like Wildwood, one in rural Iowa and one just west of Detroit, as well as, IIRC, Oil City, PA.
 
David, it seems like what was KPPC 1240 Pasadena, CA was 100 watts until the 1980s and a change in FCC rules.

The last 100 watt AM station in Michigan I recall was WBFC 1490 Fremont, MI which later moved to Whitehall, MI, now WKLQ.

I noticed that low efficiency, R. Fry, and wondered if the antenna was so inefficient that it behaved like a Part 15 type antenna, or if that was a mistake. Still, in my area, it would leave the out county areas without service, particularly at night, even assuming 8 mS/m conductivity.

I wonder what the NIF would have been. I've only seen a few rigorously calculated, but the typical Class C NIF is about 25 mV/m. It would have typically been 12.5 mV/m for the 250 watt level, and an NIF probably somewhere in the high single figures way back when there were few stations and low efficiency antennas.

Still, according to your latest calcuations, that would only be about 3 miles to the NIF. But at least no one would be staying on day power and directional pattern.
 
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