Franklin Antenna Query.

[RJ Kanary]

Back when I still could do such things, I was a frequent visitor to the KDKA 1020 xmtr site in Allison Park. As time passed changes appeared as technology improved. First the open 300 ohm feedline was replaced with an underground system, then the remaining open feedline and flat dipole utility pole(s) mounted AUX antenna was replaced with an actual tower, some time before the tower imported from Saxonburg in 1939 was brought down to be replaced.

{I am holding a piece of that tower encased in Lucite® in my hand , even as I type.}

When the new AUX stick went up that was the first time I had ever seen a ground radial system installed. [It was apparently the first time the folks that keep the greenery at the site under control had seen one too, as may radials disappeared under their rotary mowers.]

The question I'm leading up to is, the new MAIN stick has an extensive radial system installed, but I don't believe it's predecessor did.Could this apparent (To my untrained eye.) change have anything to do with their present difficulties that are keeping this blowtorch damped down ?

RJ

 

[DanStrassberg]

The question I'm leading up to is, the new MAIN stick has an extensive radial system installed, but I don't believe it's predecessor did.Could this apparent (To my untrained eye.) change have anything to do with their present difficulties that are keeping this blowtorch damped down ?

KDKA's stick, though definitely sectionalized, is not a true Franklin. It has some weired sectionalization that I don't pretend to comprehend. Therefore, it may well require ground radials. Somebody more skilled in the art than I am is going to have to answer that one.

However, a true Franklin (and the predecessor of the current KDKA stick might have been such; I don't know) does not require a ground system. A true Franklin comprises two 180-degree sections insulated from each other and fed at the midpoint.

An apocryphal story about ground systems and Franklins comes from KFBK and someone who once worked there or someone who knows someone who once worked there. KFBK has two Franklins or two sectionalized towers that are very close to Franklins--although you might not suspect it by looking at the CDBS records for the station. Supposedly, when the FCC inspector came to the newly built site to inspect it, he refused to approve it for a license to cover because the towers had no ground systems. The designers tried to explain that no ground system was necessary with these towers but the FCC guy would have none of it. Ground radials were installed and the license was granted, but according to the story, the radials were never connected to the towers.

 

[nmoore6676]

According to Scott Fybush who has seen more towers than anyone else. only KFBK, Sacramento qualifies as a true Franklin. My question is that given the radiation efficiency of this type of antenna that the high end stations and the extended band stations on AM don't use them. I can see that for low band stations that the amount of steel in the air is formidable but for high band operators it wouldn't be such a big deal.

 

[fybush]

Even at the very top of the band, a Franklin involves some serious steel. The pair at KFBK are each over 600 feet tall; at 1700, you'd still be looking at a 500-footer. There aren't many neighborhoods where you can build one of those these days.

 

[RJ Kanary]

After peering into the FCC records, I find this:

Power: 50 kW, Non-Directional
Hours: Unlimited
Pattern Type: Theoretical
Towers: 1 Augmentations: 0
Sectionalized Tower A: 115.4° B: 9.9° C: 264.2° D: 16°
RMS Theoretical: 431 mV/meter (per kW) or 3047.63 mV/meter at 50 kW

Does this imply that the radiator is actually composed of four electrically separate sections?

RJ

 

[DanStrassberg]

After peering into the FCC records, I find this:

Power: 50 kW, Non-Directional
Hours: Unlimited
Pattern Type: Theoretical
Towers: 1 Augmentations: 0
Sectionalized Tower A: 115.4° B: 9.9° C: 264.2° D: 16°
RMS Theoretical: 431 mV/meter (per kW) or 3047.63 mV/meter at 50 kW

Does this imply that the radiator is actually composed of four electrically separate sections?

RJ

This is for KDKA but you left out one very important paramter--the overall height, which IIRC is listed as something like 280 degrees. If C were 164 instead of 264, it would sort of make sense because the A and C section lengths would add up (pretty closely) to the overall height. I would then assume that B and D were "top" loads--B at the top of the lower (A) section and D at the top of the upper (C) section. I assume that these physically resemble top-hat loads on conventional top-loaded towers. But even if you were to add the electrical lengths of A and B to get the electrical length of the lower part of the tower, you would have only about 125 degrees. The upper part would be very close to 180 degrees, however. I strongly suspect that the 264-degree C dimension is in error and that a photo of the tower would reveal the two top loads. In any event, the shorter lower-section length and height of less tan 360 degrees mean that this sectionalized tower is not a true Franklin and very likely must have the ground system that it apparently does have.

 

[DanStrassberg]

Even at the very top of the band, a Franklin involves some serious steel. The pair at KFBK are each over 600 feet tall; at 1700, you'd still be looking at a 500-footer. There aren't many neighborhoods where you can build one of those these days.

At night at least, ex-band AMs are not supposed to use towers taller than 90 degrees. Some ex-banders with taller towers actually do reduce their antenna-input power below 1 kW to limit the inverse-distance field in the horizontal plane to 306 mV/m @ 1 km, which I believe is the value produced by a 90-degree tower with 120 quarter-wave ground radials. Now, it's true that Franklins have vertical radiation patterns that exhibit much less high-angle radiation than do conventional 1/4-wave towers. But if someone proposed a Franklin for an ex-band station, would the FCC allow a night power that radiated the same energy at high angles as a 1/4-wave tower with 1 kW input? The tall tower could certainly be simulated, so a vertical-radiation plot could definitely be generated. But over just what range of vertical azimuths would the FCC want you to match the radiation of the 1/4 wave tower with 1 kW input? I don't think this question has a simple answer because the answer would seem to depend on the critical angles to all of the co-channel stations.

 

[RJ Kanary]

The apparent omission of the electrical height of the radiator is not an omission on my part That parameter was not provided in the reference that I chose to use.The actual physical height is listed as 219 meters elsewhere.

RJ.

 

[rfry]

The belief that a Franklin antenna does not need a ground plane such as needed for MW monopoles may be based on thinking of the Franklin as some kind of center-fed dipole -- which dipole is a balanced radiator. The current flowing on the two halves of a dipole is equal and has opposite polarity at every instant of time. Balanced radiators don't need or use a ground plane.

But the elemental parts of sectionalized MW radiators such as the 180/180 Franklin and its variations usually are fed in phase, and with different power radiated by the two sections. This permits increasing the gain in the horizontal plane while reducing skywave radiation above 45 degrees elevation or so, compared to a 195 degree monopole. This can reduce the nighttime self-interference area, and move it further from the transmit site.

Charles Jeffers of WOAI published a paper in the Proceedings of the IRE describing the experimental studies he and others did in 1948 on a 180/120 "Franklin." They were done in reduced scale, and used a buried copper-mesh ground screen that extended more than 1/2 of a free-space wavelength from the base of the model.

NEC modeling is useful to show these elevation patterns and gains for various phases and currents in each section, and the affect of a ground plane on them.

//

 

[hogrod]

I knew of two other franklins, Binghamton NY don't know the calls and Ok City, WKY. I don't know if either of them are still in operation.

 

[RJ Kanary]

WKY is apparently alive and well, using a sectional radiator for daytime.

Power: 5 kW, Non-Directional
Hours: Daytime
Pattern Type: Theoretical
Towers: 1 Augmentations: 0
Sectionalized Tower A: 145.3° B: 15.66° C: 323.7° D: 0°
RMS Theoretical: 476.37 mV/meter (per kW) or 1065.2 mV/meter at 5 kW

RJ

 

[DanStrassberg]

WKY is apparently alive and well, using a sectional radiator for daytime.

Sectionalized Tower A: 145.3° B: 15.66° C: 323.7° D: 0°
RMS Theoretical: 476.37 mV/meter (per kW) or 1065.2 mV/meter at 5 kW

Sectionalized, yes; Franklin, no. Looks as if the tower is almost 110 degrees taller than full wave and the lower section (even with the loading provided by its "top" load) is ~20 degrees shorter than half wave. The efficiency is not quite the equal of KSTP's (almost) true Franklin.

BTW, the Binghamton NY station that somebody mentioned is WNBF 1290. It now runs 9.3 kW by day into a conventional half-wave non-sectionalized tower, achieving efficiency of 381 mV/m/kW @ 1 km, and runs 5 kW into three 97-degree non-sectionalized towers at night. I believe the daytime RMS of 1162.2 mV/m @ 1 km is the same as the station used to achieve with 5 kW from its taller, sectionalized tower. I believe that the tall tower remained in use until recently for TV, but I don't know whether it is still in use.