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Old 2011-02-06, 08:47 PM   #16
majortom
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Does anyone have any of those Elliptical or helix designs modeled somewhere?
Was lookin at that Wade site Holl_ands pointed to, specifically WH-14-69 series, and saw they claim 470 - 806 Mhz, 18.7, 19.7, and 20.7 "dBic" with only 12 loops. Is that even possible over such a wide bandwidth?
Nuther question, is how do they get it to a 75 ohm impedance?
Lastly, what the heck is dBic? (c-> circular maybe??)
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Old 2011-02-07, 09:45 AM   #17
old sparks
 
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ota canuck;

When receiving a CP signal with a linear antenna, I don't *think* it will have much effect on multipath. I do recall that CP (at least for radar) will suffer less attenuation from precipitation...

However, when receiving a CP signal with a CP antenna, reflected signals will be up to 20dB down from direct path signals.
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Old 2011-02-07, 01:41 PM   #18
Cham
 
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Wouldn't there be a 20-25% drop in reception if receiving the elliptical signal using a horizontally polarized antenna all other things being equal? I expect most of us wouldn't notice 1dB anyway..
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Old 2011-02-07, 02:04 PM   #19
tvlurker
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You're thinking about this from a point-to-point link budget point of view.

In a broadcast sense, the protection contours are based on the horizontal ERP only. So, while its true that a horizontally polarized receive antenna may not get much benefit from the vertical component , the power its missing is not necessarily "lost" from a receive point of view.

Let's say a broadcaster is limited to 30 kW horizonatal ERP in a certain direction. That's the most he's allowed to throw (horizontally-polarized) in a that direction, in order to meet protection rules. However, the broadcaster can choose to transmit a vertical component as well ( I can't remember the limit, but certainly not more than the horizontal component.) While this vertical component certainly costs the broadcaster money in terms of more output power at the transmitter, the effect on receivers is gravy -- for example, rabbit ears users will get better reception, certain multipath conditions will get better reception, but no one will get worse reception.

So it's win win, at the cost of a higher hydro bill for the broadcaster. But the broadcaster gets access to vertically polarized mobile devices, and for VHF, gets better in-building coverage to rabbit-ear users.

The user with the horizontally polarized antenna does not lose anything, because the alternative was for the broadcaster to broadcast horizontally only wih the same ERP.
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Old 2011-02-07, 05:15 PM   #20
Cham
 
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TVL,
Well worded. Then I would expect a net gain to most reception situations especially in built-up areas where the signal would have reflected components anyway. Then in a more theoretically perfect reception situation with a horizontally polarized antenna the broadcasters increased RF output would more than compensate for the cross-polarization losses. Then when is anything perfect?
I don't really see a down-side to this other than the expense to the broadcaster. Obviously they have realized the updated antennae will benefit or they wouldn't be changing things. Might be time for updates anyway to minimize failures on the old antenna & feedlines.
Interesting stuff..
-C.
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Old 2011-02-08, 01:00 PM   #21
holl_ands
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Quote:
Originally Posted by majortom View Post
Does anyone have any of those Elliptical or helix designs modeled somewhere?
Was lookin at that Wade site Holl_ands pointed to, specifically WH-14-69 series, and saw they claim 470 - 806 Mhz, 18.7, 19.7, and 20.7 "dBic" with only 12 loops. Is that even possible over such a wide bandwidth?
Nuther question, is how do they get it to a 75 ohm impedance?
Lastly, what the heck is dBic? (c-> circular maybe??)
As you guessed, dBic is relative to the gain of an isotropic antenna with circular polarization:
http://en.wikipedia.org/wiki/Decibel

In 2007, Hasan-Murtaza reported that he had built an Axial Mode Helical Antenna and gave
some dimensions, but I couldn't find any construction details or pictures...and he is no longer active:
http://www.digitalhome.ca/forum/showthread.php?p=510683

Otherwise, I have not seen anyone post a Helical, Cubical-Quad or Quad-Yagi designed for either
Hi-VHF or UHF TV Band, nor will searching reveal any designs on this forum. Suggest start by
looking at Amateur Radio designs and then carefully RESCALE to TV Band. Here's some I posted earlier:
http://www.af9y.com/helix.htm
http://helix.remco.tk/

These articles discuss how the final turn acts as an "tapered transmission line", slowly changing
from the impedance of the antenna to the desired impedance at the connector.
If you have an SWR Meter (readily available for Amateur Radio Bands....but NOT TV Bands)
the shape of the final turn can be "tuned" to minimize the SWR [For TV, use a Network Analyzer]:
http://home.earthlink.net/~w6rmk/antenna/helixmatch.htm
http://df2ck.de/tech/helix/
http://www.microwaves101.com/encyclo...rs_tapered.cfm

Helical antennas have adequate bandwidth to cover either the Hi-VHF Band (1.24 High/Los Ratio)
and (perhaps just barely) the NEW UHF Band (1.48 High/Low Ratio). The fol. MS Thesis
addressed the small Spiro-Helical Antenna (a slinky wrapped around a mailing tube) compared
to the conventional Helical Antenna, see for example Figure 4.1 on page27:
http://scholar.lib.vt.edu/theses/ava...cted/etdig.pdf
Bandwidth can be expanded by using different winding ratios for the front and back halfs or by
slowly varying the winding ratio, such as in this Tapered Helical Antenna design:
http://www.supernec.com/thelix3.htm

BTW: 4nec2 includes a BUILD/HELIX option in Geometry Editor....or use NEC's "GH" Helix/Spiral
statement by reading the NEC-2 Guide (see pg 20). Click on HELP/NEC-USER-MANUAL in 4nec2.

Analysis of Helical antennas, including an example 4nec2 file, can also be found at www.cebik.com
[Free Registration Required].
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