Franklin Antennas for MW Broadcasting

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R. Fry
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Franklin Antennas for MW Broadcasting

Post by R. Fry » Thu Aug 18, 2016 5:23 pm

One concept of the Franklin antenna is that its configuration
and performance is similar to a center-fed, full-wave, vertical
dipole installed with the lower tip of its lower arm elevated
a meter or so above the earth.

But a careful reading of the patent for the Franklin antenna shows
that both the r-f current and phase distribution along each of its
co-linear sections differ from that of a center-fed dipole.

Dr. George H. Brown of RCA Labs also discussed Franklins in
his 1936 I.R.E. paper "A Critical Study of the Characteristics
of Broadcast Antennas as Affected by Current Distribution."
He describes the Franklin as "a number of half-wave antennas
placed end to end on a vertical line, and so fed that the currents
in each element are equal, and in phase
." This configuration
is not the same as that of a center-fed dipole.

The elemental parts of sectionalized, vertically-oriented
MW radiators such as the 180/180 Franklin and its variations
are fed in phase, and often with different power radiated by
different sections. This permits increasing the gain in the hori-
zontal 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.

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 a balanced radiator. Balanced
radiators don't need or use a ground plane. But the co-linear
sections of a Franklin antenna are unbalanced radiators,
and need a good r-f ground for the same reason that a single
monopole needs one.

Charles Jeffers of WOAI published a paper in the Proceedings
of the I.R.E. describing the experimental studies he and col-
leagues 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 that real-world model. WOAI used a Franklin type
antenna for quite a few years.

There are reports that the Franklins at KFBK don't need or use a
conventional, buried radial ground system. But that would be
good to confirm. I know from good sources that other installed
Franklins such as at WHO do have, and use them.

MoM modeling is useful to show these Franklin elevation
patterns and gains for various phases and currents in each
section, and the affect of the r-f ground against which
they are driven.

COMMENG
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Re: Franklin Antennas for MW Broadcasting

Post by COMMENG » Fri Aug 19, 2016 9:58 pm

This may be the Brown paper to which you were referring.
Critical Study of BC Antennas Brown.pdf
(3.01 MiB) Downloaded 157 times
COMMENG

R. Fry
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Re: Franklin Antennas for MW Broadcasting

Post by R. Fry » Sat Aug 20, 2016 5:04 am

Yes, that's it, COMMENG. Thanks for the PDF version of it.

Here is the original patent document for the Franklin antenna.
Franklin Antenna Patent.pdf
(244.84 KiB) Downloaded 130 times

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Dale H. Cook
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Re: Franklin Antennas for MW Broadcasting

Post by Dale H. Cook » Sat Aug 20, 2016 5:43 am

COMMENG wrote:This may be the Brown paper to which you were referring.
Wow - that brings back memories. I haven't seen that article since I was studying for elements 3 and 4. I was lucky to have a good EE library in town with a complete run of the Proceedings so I could read the seminal papers.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html

W2XJ
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Re: Franklin Antennas for MW Broadcasting

Post by W2XJ » Wed Aug 24, 2016 6:19 pm

A careful reading of the patent and careful inspection of figure 3 of the patent and the comparison to figure 2 days quite the opposite.

In figure 2, Mr Franklin illustrates 3 half wave elements, the current distribution and resulting pattern. In figure 3, he is basically neutralizing the radiation of the center element. Since that is a half wave element, sections 1 and 3 are, indeed, out of phase. The illustration of the antenna in figure 3 shows the middle section collapsed and horizontal. This is the same as the typical 2M collinear antenna seen in ARRL antenna handbooks. The remaining elements are indeed out of phase where intersect but if driven from the base of the lowest element, the lower end of both elements are in phase even when center fed as a full wave vertical dipole.

R. Fry
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Re: Franklin Antennas for MW Broadcasting

Post by R. Fry » Thu Aug 25, 2016 5:21 am

W2XJ wrote:... The remaining elements are indeed out of phase where intersect but if driven from the base of the lowest element, the lower end of both elements are in phase even when center fed as a full wave vertical dipole.
A 180/180 Franklin antenna is not the same configuration as a vertical, center-fed, full-wave dipole.

The phase relation at the lower ends of the two sections of a vertical, center-fed dipole differs from that of the Franklin.

Below is a clip from the Franklin patent document, showing that its co-linear sections are in phase (see the last paragraph).
Attachments
Franklin Antenna Patent (clip).jpg

W2XJ
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Re: Franklin Antennas for MW Broadcasting

Post by W2XJ » Thu Aug 25, 2016 3:59 pm

Richard,

Re-read what you pasted. It proves my point.

R. Fry
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Re: Franklin Antennas for MW Broadcasting

Post by R. Fry » Thu Aug 25, 2016 4:42 pm

W2XJ wrote:Richard, Re-read what you pasted. It proves my point.
Robert -- It doesn't prove the contention of some that the configuration and operational characteristics of a 180/180 Franklin are the same as that of a vertical, center-fed, full-wave dipole with the lower elevation of its lower arm located a meter or so above the surface of the earth.

Equal r-f currents leave the center feed-point of a dipole into both of its sections, but those currents are out of phase by 180 degrees.

In a Franklin, the r-f currents at every location along the lengths of each of its co-linear sections are IN phase.

W2XJ
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Re: Franklin Antennas for MW Broadcasting

Post by W2XJ » Thu Aug 25, 2016 7:59 pm

Richard

Please read for comprehension the patent you posted. It very clearly states that the middle section of a 1.5 wavelength antenna is supressed from radiating. The remaining sections follow the current distribution of of a center fed diopole. The KFBK array just removes the instability of many sectionalized antennas by eliminating mid tower phasing elements.

R. Fry
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Re: Franklin Antennas for MW Broadcasting

Post by R. Fry » Fri Aug 26, 2016 5:34 am

The performance of a 180/180 Franklin and a full-wave, center-fed, vertical dipole are not the same (see below).
.
Performance Comparison, Franklin Ant.jpg

Kelly
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Re: Franklin Antennas for MW Broadcasting

Post by Kelly » Fri Aug 26, 2016 8:21 am

KNBC used to run a Franklin too: http://bayarearadio.org/schneider/radio019.shtml
Skype:kellyalford Twitter: @KellyAlford

W2XJ
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Re: Franklin Antennas for MW Broadcasting

Post by W2XJ » Fri Aug 26, 2016 3:38 pm

KNBC was never a Franklin. It is a sectionalized radiator shorter than most radiators used by class A stations.

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kkiddkkidd
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Re: Franklin Antennas for MW Broadcasting

Post by kkiddkkidd » Fri Aug 26, 2016 6:04 pm

IIRC, WKY OKC has an extremely tall Franklin.
--
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AM Ground Systems Company
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BigRed
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Re: Franklin Antennas for MW Broadcasting

Post by BigRed » Fri Aug 26, 2016 6:16 pm

KFBK-AM in Sack-O-Tomatoes, CA (Sacramento, CA) had a two-tower DA that consisted of Franklin radiators at one time, maybe they still do? (Do they need a 4-tower phasor for that ? ? ? Never mind . . . )

http://www.fybush.com/sites/2005/site-051028.html

They also had a 50-kW Amplifuzz transmitter. What could go wrong with THAT set-up? :D

They had an advantage in that they are at the upper end of the dial. If you want to build one down around 700-kHz go for it. Just let me know before you start ordering towers as I want to shift some of my investments into steel.

Kelly
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Re: Franklin Antennas for MW Broadcasting

Post by Kelly » Sun Aug 28, 2016 9:53 am

W2XJ wrote:KNBC was never a Franklin. It is a sectionalized radiator shorter than most radiators used by class A stations.
Yes it is and I believe it's still in use: http://lists.contesting.com/_topband/20 ... 00197.html

https://plus.google.com/+OlegZabluda/posts/UhDRr9R1ZgN

http://radio-timetraveller.blogspot.com ... -talk.html

Back in the 70's I toured the site with my uncle, who had explained the theory of the Franklin design at MW frequencies.
Skype:kellyalford Twitter: @KellyAlford

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