One has to be careful with 5/8 wavelength verticals. A radiator that is
physically 5/8 wavelength is already electrically too tall. That is why
a 300 foot BC tower would not work well at low angles on 160. There are
too things to considers one is that towers have velocity factor just
like coax and the other is guy wires and anything else attached to the
tower will have a loading effect. The size of the tower face also has an
effect.
I knew someone who without doing the necessary engineering built a 225
degree BC radiator and had horrible results because he did not take the
above factors into consideration. It is difficult to say what a safe
physical height might be without fairly precise modelling. A significant
number of 50KW former clear channel stations use 195 degree radiators.
Part of the logic is that above that height a minor high angle lobe
becomes significant and causes sky wave cancellation of the ground wave
which is a concern to broadcasters. But the other point is that 195
degrees is far enough away from 225 degrees that the mechanics of the
install is not important unless that tower is also supporting some beam
antennas.
There is one true Franklin on the BC band in Sacramento CA. There are
several other sectionalized radiators in service but the generally tend
to be high maintenance.
On 5/6/12 5:07 PM, Herb Schoenbohm wrote:
> In 2006 Tom Rauch, W8JI mentioned the disappointment with 3/8 wave
> vertical antennas and Carl mention today abut how "BCB stations migrated
> from 5/8 wave and 1/2 wave antennas." I added to Tom's rejoinder that
> several AM stations spent considerable amounts of money with the
> Franklyn design which was claimmed to lay more radiation at lower
> angles. This is possible if the two is insulated and a phasing device
> is place between the upper and lower tower sections. Presumably it can
> be accomplished even with reduced height or a squashed design of the
> true Franklyn. Admittedly I have yet to hear of any TB'er to use this.
> However a 3db signal enhancement at low angles in all directions may be
> something to consider. I would also wonder if putting to much RF below
> the critical angle (since DX-ers) are not particularly interest in
> ground wave coverage and need sky wave instead) would be detrimental.
> There are times when a higher angle take off is the difference between
> being heard or not especially, I think, during SR/SS Grey line
> enhancements, and maybe on some skews and spotlights. I post the
> Franklyn information just the same for those who may have missed the
> original post.
>
>
> Herb Schoenbohm, KV4FZ
>
> Quoting Tom Rauch<w8ji@contesting.com>:
>
>
>> / Some of the biggest failure antennas I have used were 5/8th/
>> / wave verticals at broadcast stations. We loaded one AM tower/
>> / that happened to be a 5/8th wave on 160, and it was poor/
>> / compared to a short vertical./
> The balloon lengths has increased my curiosity in learning what principles are
> working here. Theoretically, very low angle radiation could be obtained by a
> balloon supported long wire with "controlled current distribution". (ARRL
> Antenna Compendium Vol. 2 pp. 132-135)
>
> As I mentioned before in my case the 5/8 vertical 308 foot insulated tower,
> totally surrounded by sea water was a big disappointment on 160 meters. I
> tried it for 5 years and the lower antennas were always noticeably better.
>
> I once worked for KUOM which shared a tall tower with KSTP 1500 kHZ in
> Minneapolis. Stan Hubbard, owner of KSTP was convinced to erect a Franklin
> antenna design which was supposed to modify the current distribution on tall
> towers to lay out a stronger ground wave then the 1/4 wave or smaller AM
> radiators. All the theory, the engineer and construction cost, sort of like a
> Ringo Ranger for the broadcast band were very disappointing. Years of A/B
> testing driving across the Dakotas, WCCO (although lower in frequency) was the
> king of signals from the Twin Cities by a significant margin. Both were 50KW
> clear channel stations. (KSTP bragged 100KW Effective Radiated Power) Some
> claimed
> this was due to sky wave and ground wave out of phase arrivals in which case
> the
> Franklyn actaully redued the sky-wave component, at least in theory.
>
> The Franklin concept can be found in Jasik's First Edition Antenna Engineering
> Handbook pp. 4-35 and 4-36. A traditional Franklin was two half waves stacked
> end to end and fed in phase. KNBC (Los Angles)built one in 1949 as a means
> of lowering the angle of radiation, but used a 550 foot tower since at 680 Khz
> a true Franklin would have been 1500 feet tall. They were apparently able to
> design a much shorter structure since their top portion was top loaded with a
> capacity hat and only 150 feet tall. (Put "KNBC Franklin Antenna" in your
> search engine for some awesome pictures of this antenna.) Did it actually
> improve coverage for KNBC? Are they still using it today?
>
> It would be interesting to learn if any AM stations still use the Franklin
> design and if
> the shortened Franklin (ala KNBC) has any merit for consideration on 160
> meters as a shortened gain low angle DX antenna As far as I have been able
> to find out, collinear verticals below VHF are just not worth the effort, but
> that is not what the books tell us. Yet in practice a 1/4 to 3/8 wave appear
> to be the best topband performers for all the reasons stated in
> previous posts. (The 3/8 wave if converted to an Inverted L was popular in
> the 60's
> as it provided a 50 to 60 ohm feed point with just some inductive reactance
> to tuned out
> to actually resonate the wire as a 1/4 wave. In an inverted L configuration
> there is
> radiation in both the horizontal and vertical portion. I mention this since
> this would
> be a totally different antenna then a bottom feed 3/8 wave vertical tower.)
>
>
>
> Herb Schoenbohm, KV4FZ
>
>
>
>
>
>
>
>
>
> On 5/6/2012 12:31 PM, ZR wrote:
>> The BCB stations migrated from 1/2 and 5/8 wave antennas, diamond shaped
>> towers, and mountain tops by the early to mid 30's as they started to
>> understand how things worked...or didnt.
>>
>> Carl
>> KM1H
>>
>>
>>
>>
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> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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