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Author Topic: Franklin antenna VS Vertical dipole  (Read 2746 times)
N3JBH
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Posts: 2358




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« on: November 26, 2008, 03:46:07 PM »


Dear Elmer’s any of you ever had experience making a Franklin antenna?
Was wondering the benefits over a vertical dipole? I am considering a low noise type vertical antenna to compliment my Horizontal dipole for 17 meters. Any wisdom advice or suggestions are most welcomed. Criticism gladly accepted as well.  Jeff
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N4JTE
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« Reply #1 on: November 26, 2008, 04:03:40 PM »

Okay i'll jump in, : What the hell is a Franklen antenna, something on a kite string?
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VK1OD
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« Reply #2 on: November 26, 2008, 04:21:02 PM »


Much of the ambient noise comes from quite local to you, at very low elevation.

Increasing the gain of an antenna at low angles, emphasises the noise wrt higher angle sky wave reception.

The Franklin antenna is intended to emphasis gain at lower elevations.

Now, explain the concept of a "low noise vertical", and how the Franklin fits.

Owen
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WA7NCL
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« Reply #3 on: November 26, 2008, 04:29:43 PM »

According to some info on the internet.  Franklin is two half waves colinear array mounted vertically.  It would be twice as tall as a vertical dipole (half wave) and theoretically have a lower angle of radiation over perfect ground.  With real ground, its not likely to come any where near theory, especially at 18MHz.

Such a system is easily modeled in EZNEC.  Add the various ground types to your model and you will see what I am talking about.
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N1LO
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« Reply #4 on: November 26, 2008, 04:41:03 PM »

Hi Jeff,

Following up on NCL's description (I didn't know what a Franklin antenna is either!), I can think of something similar. I built a 2 x 5/8 wave wire collinear for 10m. See w w w dot qsl dot net/N1LO.

In my 2 meter experiments, I was not very happy with two 1/2 waves in phase as a vertical collinear. The 10m super J, with 2 x 5/8 waves, compared very favorably with my C3E beam.

Think of it as an extended double zepp (a killer horizontal wire) turned vertically and fed from the bottom.

Scale up the plan by the ratio of frequencies: 28.4/18.140. It'll be very tall, if you have the height.

Have fun!

--...MARK_N1LO...--
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N3JBH
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« Reply #5 on: November 26, 2008, 04:55:28 PM »

Well Owen honestly  thought a franklin was grouned at the base there being not certian of them was hoping it would maybe drian static Huh
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W8JI
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« Reply #6 on: November 26, 2008, 05:33:44 PM »

Jeff,

Static cannot be "drained". It is noise just like a radio signal, so anything that shorts the antenna to ground for noise at the operating frequency also shorts it for the desired signals.

Now it is possible in really bad weather to have charges build up on the antenna and arc over at weak points, and a ground path will stop that.

A ground path will NOT, in my experience and the experiences of many others with tall antennas, reduce corona noises. This is because the voltage gradient that causes corona is between the clouds and the antenna, and the ground can't possibly decrease that voltage.

So I'm afraid you are stuck.

There is an additional problem with the Franklin. Low wave angle losses are very high with a vertical. This is because of the soil out some distance from the antenna. If you compress the signal along the earth you can actually decrease field strength.

Now I know some people use 5/8th wave antennas or combinations, but if you look at the 5/8th wave it actually increases higher angles. There is no Fresnel region loss at higher angles.

If I was going to build a HF vertical, I'd either mount it way up high or make it a design that had a little bit higher radiation angle. You really don't want a zero degree focus because that is where all the loss and all the noise is.

73 Tom
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W4VR
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« Reply #7 on: November 26, 2008, 06:04:32 PM »

Franklin antennas were popular in the AM broadcast industry years ago.  You're lucky to find one today.  There are better antennas you can put up.
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N3JBH
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« Reply #8 on: November 27, 2008, 05:02:14 AM »

Hey Thanks every one. I figured there had to be a reason why it was not used much. And thanks Tom i guess i was having a senior moment. If would really thought about this instead of running half cocked with another wild idea I realized that. Well Happy Thanks Giving folks Jeff N3JBH
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WB6BYU
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« Reply #9 on: November 27, 2008, 09:29:21 AM »

The Franklin Uniform aerial was simply a way of folding a wire so that
most of the radiation was in phase.  The nearest modern equivalent is
a quarter wave "phasing stub" separating two half wave radiators.  The
idea was that, rather than having the stub stick straight out where it
makes little contribution to radiation, it was actually twisted around
so that the portion with the highest current was flowing in the same
direction as the rest of the wire.  Basically at the end of a half wave
section the wire goes 1/8 wave further, folds over and goes back down
1/4 wave, then back up 1/8 wave to the bottom of the next radiator
section.  

If you are modeling it you will want to make sure that all the segments
line up in the parallel wires.

While I always enjoy experimenting with some of the older antenna
designs, and this certainly would be a conversation piece, the simple
3/4 wave antenna with a quarter wave stub probably is easier to build
and adjust (via the stub length) and, at least with a cursory attempt at
modeling, worked a bit better.   But if you want a simple way to fold
a length of wire into a colinear antenna without having stubs sticking
out, this would be an option.  While it was originally designed as a
vertical antenna, it can also be used to make horizontal colinear arrays.
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AC5UP
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« Reply #10 on: November 27, 2008, 01:00:37 PM »

Speaking of...

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

50 gallons on 1530 kHz with a claimed groundwave field strength of 3545.89 mV/m --- Highest of any US AM broadcast station.

Something to be said for that. Wink
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N3JBH
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Posts: 2358




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« Reply #11 on: November 27, 2008, 01:05:14 PM »

WB6BYU Thanks and yes you pretty much described what i had in mind. But after Tom's write up i pretty much forgot the whole idea. I really respect his words of wisdom. Him and i am sure a whole lot more of you have proballby Forgotten more this morning then i ever learned in a life time. I am just thrilled that all of you are so willing to hear my hair brained ideas and offer folks like me guidance,  Happy Thanksgiving Jeff
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NA6M
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« Reply #12 on: February 10, 2009, 02:38:55 PM »

I worked there for 14 years. The station is referred to as the flame thrower of the central valley. Another point of interest, KFBK installed serial number 1 of the Harris DX-50 solid state transmitter in 1989.
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