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Antenna bandwidth vs VSWR

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LakeMan2

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Feb 21, 2014
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Usually nothing is for free and there are tradeoffs. Yet for the dipole's I have been looking at, Sinclair's have 106 Mhz bandwith and a DB-408 has 20Mhz both with the same VSWR spec (<1.5). I am surprised by that. They are both folded dipole and I would think that any patents have long since expired. If 106 Mhz was possible with no tradeoffs, I would think the DB-408 would do it if for nothing else than marketing purposes. If there is some benefit to a design with a tighter bandwidth, I don't see it in the DB-408 specs.

I could see it if it were reflected in the VSWR spec (higher VSWR spec for a wider bandwidth), but that does not appear to be the case.

Any ideas?

Thanks.
 

prcguy

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Not sure if this is the reason but the Sinclair appears to be an actual folded dipole where the DB Products is not. If you look closely at the DB-408 and its cousins the bottom loop is simply a 1/4 long folded element with both ends grounded to the support boom. Its not much different than a simple 1/4 wave element except there are two of them in parallel and the band width would be wider than a single length of tubing of the same diameter.

The top loop on the DB-408 is a folded monopole with one fed near the grounded boom then it goes up for about 1/4 wavelength then back down to ground. The folded monopole is not much different from a simple 1/4 wave vertical but it does seem to have more band width and it is grounded, which is good for lightning purposes. DB Products and others make a low end 1/4 ground plane that uses the same folded and grounded 1/4 wave type element.

The Sinclair dipoles appear to be an actual loop but not fed like a typical folded loop element like you would find on a Yagi or the twinlead dipole that came with your stereo receiver, those are around 200 ohms impedance in free space. The Sinclair elements are fed with 50 ohm coax but have a length of 125 ohm coax inside the element as part of a matching transformer and I don't know what their actual feed point impedance is but its probably higher than 50 ohms. I believe an actual folded dipole like the Sinclair has a wider BW than the dipole configuration on the DB Products.
prcguy

Usually nothing is for free and there are tradeoffs. Yet for the dipole's I have been looking at, Sinclair's have 106 Mhz bandwith and a DB-408 has 20Mhz both with the same VSWR spec (<1.5). I am surprised by that. They are both folded dipole and I would think that any patents have long since expired. If 106 Mhz was possible with no tradeoffs, I would think the DB-408 would do it if for nothing else than marketing purposes. If there is some benefit to a design with a tighter bandwidth, I don't see it in the DB-408 specs.

I could see it if it were reflected in the VSWR spec (higher VSWR spec for a wider bandwidth), but that does not appear to be the case.

Any ideas?

Thanks.
 

teufler

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Dec 19, 2002
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a b&w has a band width from 160 meters to 6 meters, under 2.0 swr. Used at field day alot, a bear to roll up and kleep the wires untangled for the nerxt use though.
 

lmrtek

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Feb 11, 2009
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I also find the Sinclairs to be better for extreme environments because the feed points are well sealed and the harness is inside the support pipe

A true folded dipole is a very broad band antenna and
That's why TV antenna makers have used them for many decades

While the monopole design multi bay antennas are better for lightning survival, they sacrifice bandwidth

Comprod antennas also use folded dipoles and their entire line from vertical stacked dipoles to wide band Yagi's and corner reflectors
 
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Texas
I also find the Sinclairs to be better for extreme environments because the feed points are well sealed and the harness is inside the support pipe

A true folded dipole is a very broad band antenna and
That's why TV antenna makers have used them for many decades

While the monopole design multi bay antennas are better for lightning survival, they sacrifice bandwidth

Comprod antennas also use folded dipoles and their entire line from vertical stacked dipoles to wide band Yagi's and corner reflectors
Most people don't actually realize that DB antennas were initially only designed to have an 8 year service life. They were designed around a (relatively) low price point with a decreed service life in mind. It was intended the antennas could be taken down and rebuilt and retuned with a new phasing harness in less than a day in comparison to the Sinclair and Telewave counterparts which you don't really want to take down once you get them up due to their size and cost.
 
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