My understanding is that a half wavelength long feed line presents zero
impedance transformation. See the Smith chart -
http://www.scott-inc.com/img/smith.gif
The idea being that if you have an impedance of anything other than feed line's
impedance ("a perfect match" represented by the point marked 1.0 in the center
of the chart) and you plot that impedance it will be a certain distance and
direction from 1.0. Then using a compass you measure from 1.0 on the chart out
to the plotted input impedance point and swing an arc and draw a circle
centered on 1.0 that intersects the plotted input impedance. To calculate the
impedance seen at any point along the feed line as you move down the feed
line's electrical length (fraction of a wavelength) you move around the Smith
chart (actually around the circle you drew) and that will be the impedance seen
at that point on the feed line. The scale around the outer diameter of the
Smith chart reads in decimal fractions of a wavelength. If you go 1/4 wave down
the feed line that represents going .25 wavelength around the Smith chart's
outer scale which corresponds to going 180 degrees around the chart (the point
opposite of the input impedance on the circle you drew). If you go a half wave
down the feed line then you go 360 degrees around the circle that intersects
the feed point impedance, in other words you return to where you started.
Greg
On Jun 7, 2010, at 9:59 PM, Mike wrote:
> The whole idea of a passive repeater intrigues me. Two times in many years
> I have done just that with limited success.
>
>
>
> The first was a metal building I built for Daystar Communications in SW
> Florida. It was our NOC and housed our customer support team as well as the
> techs. Cell phone coverage was the pits. What I did was point a Yagi at a
> known cell tower a few miles away. The feed line penetrated the building
> and fed a half wave dipole. One of the benefits of that particular time in
> my life is I had access to a very nice network analyzer. The dipole was cut
> very precisely, and the feedline, LMR 600 if I remember correctly, was cut
> to a multiple of ¼ wave and acted as an impedance repeater. In that way any
> matching errors to the feedline were negated. It gave cell phones in the
> building a couple bars and made usage possible.
>
>
>
> The second one was for a customer here in Iowa. They live down in a bowl
> and couldn’t see my tower 2 miles away. They have a campground. Cell
> phones don’t work well at all in the bowl. There is a pasture which has a
> hill that rises up from the bowl. From that hill you can see my tower.
> They planted a telephone pole and ran electricity to it. We put a panel
> pointed at my tower and a second one lower as a repeater which termed the
> entire property into a hot spot. It works well.
>
>
>
> We took 2 long commercial 800 MHz Yagis and connected them together with a
> short feedline measured, with the velocity factor to be a multiple of ¼ wave
> again. One Yagi points at a cell tower, the other points at the campground.
> It gives cell phones a couple bars where they didn’t work most of the time
> before.
>
>
>
> If you used a couple high gain, efficient dishes and separated them with
> minimum feedline or hardline, it should work in a similar way. I would be
> curious to see the results as I haven’t done it with frequencies over 800
> MHz. I wouldn’t look for any magic results but reasonable results if your
> engineering is sound.
>
>
>
> Friendly Regards,
>
>
>
> Mike
>
>
>
> Mike Gilchrist
>
> Disruptive Technologist
>
> Advanced Wireless Express
>
> P.O. Box 255
>
> Toledo, IA 52342
>
> Mike's
> <http://www.tamatoledonews.com/page/category.detail/nav/5001/Local-Columns.h
> tml> Weekly Column
>
> 239.770.6203
>
> m...@aweiowa.com
>
>
>
> _____
>
> From: wireless-boun...@wispa.org [mailto:wireless-boun...@wispa.org] On
> Behalf Of Jack Unger
> Sent: Monday, June 07, 2010 5:21 PM
> To: WISPA General List
> Subject: Re: [WISPA] Repeater
>
>
>
> For this to work, one end needs to be very short. In a typical mountain-top
> repeat situation, the combined free-space path loss from BOTH paths is more
> than enough to prevent the link from working.
>
> Greg Ihnen wrote:
>
> Actually I've done this on ships where the deck department needed to
> communicate with the engine room and personnel down in compartments where
> winches for line handling were located. We're talking very short distances
> (less than the length of the ship - around a 1000 ft) and short cable runs.
> But it did let enough RF leak into the below deck areas to facilitate
> communications.
>
> Greg
>
> On Jun 7, 2010, at 4:34 PM, Bob Moldashel wrote:
>
>
>
> Garbage......
>
> Let's say optimum consideration here...
>
> Present RSL -68 db ....Subtract cable loss -2 dB = -70 Add +24 db
> for the Grid = -46
>
> Free Space Loss at 1/10th of a mile is -84 db
>
> Soooooo...
>
> If you take the -46 dB level out and add the FSL of -84 dB that will
> give you a -130 dB.
>
> I don't think that will work......
>
> Get a repeater or get a stronger receive signal at your receive
> antenna. Like -20dB
>
> -B-
>
>
>
>
> Steve Barnes wrote:
>
>
> Ok I have never even thought about doing this. Does it actually work? This
> sounds WAY to simple.
>
> A 29Db Grid on a Grain Leg pointed at the AP that has a -68 signal plugged
> into a 24 DB Grid Pointed to the house 1/4 mile away. What kind of signal
> would you have on the back side at the house?
>
> Steve Barnes
> RC-WiFi Wireless Internet Service
>
>
> -----Original Message-----
> From: wireless-boun...@wispa.org [mailto:wireless-boun...@wispa.org] On
> Behalf Of Rubens Kuhl
> Sent: Monday, June 07, 2010 4:20 PM
> To: WISPA General List
> Subject: Re: [WISPA] Repeater
>
> One option to consider is a passive repeater. Wire a coax cable between the
> two dishes and you are done... no electronics to fail, no power to supply on
> a remote location.
>
> (haven't tested this trick with dual polarity, though)
>
>
> Rubens
>
>
> On Mon, Jun 7, 2010 at 11:00 AM, Steve Barnes <mailto:st...@pcswin.com>
> <st...@pcswin.com> wrote:
>
>
>
> I have avoided repeaters like the plague but I have a situation where I have
> one and I am looking for a better option. When I started my wisp I was 100%
> Tranzeo. At this one location I setup a CPE connected to a TR-6000 that has
> 2 Ethernet ports that pass through POE. I ran 1 Ethernet up the tower with
> a POE at the bottom, and a crossover in between.
>
> I would like a similar layout for other locations. Issue I see is that not
> many other units, UBNT or MT have a 2nd Ethernet that pass through POE?
>
> How does everyone you get around this?
>
> Trying to stay cheaper than a RB433, 2 radios, and 2- antennas, box,
> pigtails, 2 LMR cables.
>
> Steve Barnes
> RC-WiFi Wireless Internet Service
>
>
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> --
> Jack Unger - President, Ask-Wi.Com, Inc.
> Network Design - Technical Training - Technical Writing
> Serving the Broadband Wireless, Networking and Telecom Communities since
> 1993
> www.ask-wi.com 818-227-4220 jun...@ask-wi.com
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