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Old 16th June 2005, 04:12 PM   #21
homer09 is offline homer09  Canada
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Quote:
Originally posted by dnsey


The best feeder for a simple ribbon folded dipole, however, is 300 Ohm.


what do you mean by feeder? the cable that connects from the antenna to the tuner?

you say a folded dipole is 300 ohm. isnt it the cable that determines what ohm it is? so to make a folded dipole with coax is inefficient? only a single dipole would be efficient with coax?
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Old 16th June 2005, 04:33 PM   #22
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I got curious and connected a J-pole to my Sansui TU-717.

*WHOA!*

The whole band came alive! I can easily get stations from 80+ miles away.. 104.7 (some 70+ miles south) sounds like it's in my backyard!

I'm impressed,and it was easy to make with some copper pipe.
If you scroll down to the bottom of this page,there's some info.
It's also possible to make a J-pole from 300-ohm twinlead.
http://www.pcs-electronics.com/en/gu...p?sub=antennas
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Old 16th June 2005, 04:43 PM   #23
dnsey is offline dnsey  United Kingdom
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Yes, feeder is the cable - it's usually thought of in terms of a transmitting antenna, hence the term.

The termination impedence of the antenna is fundamentally determined by the position of the standing waves on the element relative to the point of termination - don't worry about it unless you want to get heavily involved in antenna design

The characteristic impedence of the feeder cable should match the antenna termination in order to avoid both losses and internal reflections in the cable, which can cause distortion of the signal.

Naturally, the tuner input should match the cable.

So yes, a single dipole would be a good match to 75 Ohm feeder. If you wanted to use this cable with other designs, various techniques (Pawsey stubs, baluns etc.) can be used to match it, but there's little point in your case, as you have both 75 and 300 Ohm inputs, and can cater for either case.

There's little to choose between the two for domestic use - balanced feeder is more susceptible to impulse interference in some cases, but if you don't have trouble from motor ignition systems, switching transients etc., then this isn't important.
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Old 16th June 2005, 04:46 PM   #24
homer09 is offline homer09  Canada
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Quote:
Originally posted by DigitalJunkie
I got curious and connected a J-pole to my Sansui TU-717.

*WHOA!*

The whole band came alive! I can easily get stations from 80+ miles away.. 104.7 (some 70+ miles south) sounds like it's in my backyard!

I'm impressed,and it was easy to make with some copper pipe.
If you scroll down to the bottom of this page,there's some info.
It's also possible to make a J-pole from 300-ohm twinlead.
http://www.pcs-electronics.com/en/gu...p?sub=antennas

interesting, what design did you follow?
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Old 16th June 2005, 04:47 PM   #25
dnsey is offline dnsey  United Kingdom
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Note how the j-pole uses a variable stub to achieve correct matching to the feeder
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Old 16th June 2005, 11:12 PM   #26
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Never really looked at the J-pole. Seems rather tasty! Think I might build one. Probably use 1/2" pipe to get the bandwidth up.

Googling for J-pole turns up lots of info.
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Old 17th June 2005, 05:09 PM   #27
Rahul is offline Rahul  India
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HI!

Now this is something from deep down my heart. First you need to decide wether you need a directional antenna or an omnidirectional one.

For a directional one its simply a matter of choice either go in for a yagi or a cubical quad. All would give a gain over normal pair of dipoles but then they are directional. The gain increases with each additional element. It is a simple way to achieve a gain this may not be of significance where reception is concrened but when you transmit a 3db gain on antenna would alone provide you with signal equivalent to twice the power. This is economical when you think in terms of putting a signal worth 2KW while using only 1Kw. Passive gain.

Most antennas are restricted in bandwidth to over come this shortcoming one can try a log periodic.

A normal dipole has a feedpoint impedence of75 ohms folding a dipole cause its impedence to increase as well as its bandwidth:

Z (imp)=sq *turns

Hence a folded dipole has 2sq impedence ie 4 times that equates to 300ohms for nominal impedence of 75ohms half wave dipole.
Secondly it is balanced sort of antenna hence a balanced ribbon feeder.

Now on to omni directional one, basic one is quarter wavelength ground plane, this results in good match to various TX and signal. Going one up we come to antennas in range of 5/8 lmbda ie wavelength, the J pole comes in this catiegory these provide some gain over the basic Ground plane or the quarter wavelength.

Many may have noticed a vertical dipole .Very popular with pager systems mostly many of them are stacked to achieve some omnidirectional gain a 4 element vertical dipole provides more than 6dbs of omnidirectional gain. Once again the folded vertical dipole provides a higher bandwidth.

However a multi element qubical QUAD with an antenna rotor still beats anything available worldwide.

Now for some interested in an RF preamp do a google on norton noiseless feedback preamp or a gasfet preamp. These are hard to beat where reception is concerned.

Regards

Rahul
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Old 18th June 2005, 10:34 AM   #28
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This is all fascinating!

I'm looking at building a DAB antenna in the near future, are there any good general design reference sites that you guys can recommend?
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Old 18th June 2005, 01:24 PM   #29
dnsey is offline dnsey  United Kingdom
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I don't know of any good websites (I'm sure there are some!), but the RSGB publishes some excellent books on the subject.
Try
here
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Old 18th June 2005, 02:12 PM   #30
Ron E is offline Ron E  United States
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The proper length for a folded dipole antenna is:

L=vf*c/f
Vf=velocity factor, c=speed of light, f=frequency

For the FM band, a folded dipole has a wide enough bandwidth that tuning it to 98MHz is good enough.

For some twinlead, vf=0.75, but it can vary greatly with construction, IIRC.

L=0.75*300000000/98000000=~2.3 meters

I highly doubt that a manufacturer would make a folded dipole the wrong length, so doubting them is not productive. IF the folded dipole doesn't work for you, you may just need to orient it properly. Dipoles have a null in their gain plot very much like a dipole speaker, highest gain is normal to the plane of the antenna's "arms" and lowest (zero) gain is looking down the cable of the arms.

J-poles are omnidirectional and may pick up a lot of multipath in big cities IIRC.

I experimented with a few antenna types over the years. I finally broke down and bought one of those amplified antennas and it doesa fairly good job in the metro area where I live.

One thing I noticed when playing around wiht antennas is that there is a pretty foolproof way to check the quality of the signal. If you connect a small speaker between the + connections of the left and right channels of your amp, the speaker plays a L-R, or difference signal. I believe FM is transmitted with a L+R and L-R signal and the L-R is susceptible to degradation. Regardless of the reason why, though, if the L-R signal sounds good (no static or dropouts) The FM station will sound good.

The best antennas by far are the old TV type that have an FM section - the type that mount on your roof and need to be rotated.
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