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Discussion Starter · #1 ·
I just received my Samsung SIR-T150 from Vanns.com yesterday .... couldn't resist getting it for $349 (after the rebate). I'm just mesmerized by the HD images. After lurking here for a while, here's my first posting.


Currently, I'm just using a simple indoor antenna, and I'm able to pick up with no problem KNTV, KPIX, KQED, KICU, and KSTS. But for the life of me, I cannot get KGO (ABC) or KTVU (Fox). As far as I know, their transmitters are located on Sutro Tower along with KPIX and KQED's. Are their signals weaker than the two on Sutro I can receive?
 

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Discussion Starter · #2 ·
Like a lot of newbies, I should do a better search before I ask a question. I found an earlier post where Larry Kenney gave the ERP's of the various stations. KQED and KPIX have ERP's of 777 kW and 1000 kW respectively. The stations I can't tune to, KGO and KTVU have ERP's of 561 kW and 501 kW. I also am not able to receive two other stations on Sutro, KBWB and KBHK, which have ERP's of 383 kW and 206 kW. With the cheap set top antenna I am currently using (I found it in my "electronic junk box"), the signals of the station I am able to receive are pretty weak, so that may be the problem.
 

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Ri-Chee,


You may want to visit your local Rat Shack and get their double bowtie antenna. However, it won't work for KNTV-DT - VHF. Some folks have modified the bowtie and removed the 300 ohm lead and replaced it with a 75 ohm coupler right at the connectors on the antenna. Reports indicated a vast inprovement in signal quality and level.
 

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Im up here in Marin and I get KQED/KGO/KRON but no KPIX and KTVU. Some of the stations I get are sent out with less power than others I receive via a 4228Bowtie. I spoke with a guy from KPIX and he said there transmitter is pointed slightly down to cover the city. SO, I believe in my case, and maybe yours, it might have more to do with directionality than juice. I have no answer about KTVU other than a engineer telling me if I get KRON I should get theres as they are located next to KRONs on the tower. Its a crap shoot!!!
 

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Discussion Starter · #6 ·
I stopped by a couple local Rat Shacks, but neither of them carry the double bowtie antenna anymore. The last one I was at was right next to a Circuit City, so I begrudgingly set foot inside and picked up the Terk TV-20, just to see if that antenna makes any difference. I'm happy to report that I can now receive every SF Bay Area digital transmission. No need for an external antenna :)
 

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Congratulations! Welcome to the DTV paradise that is the Bay Area.


(Myself, I use the double bow tie and pick up all the local digitals, including KNTV. It may be a UHF antenna, but it's just enough metal to grab that VHF signal.)
 

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Am here at SSF in the peninsula. I have a Sony 36xbr450. RCN is my cable provider. I am looking at getting a HDTV decoder. Any recommendation on what to get. I want to be able to hook up my cable onto it and i guess an antennae to be able to receive HD local broadcast. What can you guys recommend...Something in the range of 400-600usd or even less. I looked at the Samsung SIR-T150 and it doesnt say about cable connection tho. ..Really appreciate any input from you guys...THanks a lot.
 

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No set top box can accept input from a cable TV operator, unless the operator uses the 8VSB transmission standard.


RCN doesn't provide any high definition digital television programming anyway. So, plug your cable TV decoder into one input on your TV, and your set top box into another input on your TV, and enjoy.


I've got the SIR-T150 and it's great at bringing in even weak signals. I also just got the MyHD card for a home theater PC and it seems to work pretty well, too (still testing it out).
 

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I'm in Mountain View, use a RS Double Bowtie as well and receive everything around the bay including KNTV (just like SeanKelly, but I need to move the antenna a bit to get certain stations). I just picked up the part to do the modification and hopefully I can leave it in one place after that.


As for a receiver, I use a Sony HD100 and it will handle a cable input, a DTV input and an over the air input all at the same time. They're pretty expensive and have been discontinued, but you might be able to find an open box or a used one. They have their share of issues (like all of the STBs I've seen), so do some research before you buy.


- Peter
 

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Discussion Starter · #12 ·
Can someone who has one tell me the catalogue number for the RS Double Bowtie Antenna? I'm just curious about it. Thanks.
 

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Hurry, Ri-Chee! The 15-623 antenna has been officially discontinued. You might have to drive around a bit to find one still collecting dust on the shelf somewhere.
 

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Discussion Starter · #16 ·
Hi Sean,


Thanks for your welcome in an earlier reply. The Bay Area does seem to be a great place for OTA HDTV and DTV.


I was asking about the double bowtie more out of curiousity (since my Ph.D. and supposed professional area of expertise is in electromagnetics/antenna), to find out a little more about the antenna since it works so well for so many people in the valley here. I tried searching for it with the description "double bowtie antenna" on RS's website and couldn't come up with anything, so I wanted to try searching with the part no.


I returned the Terk TV-20 to C.C. and bought the new Terk TV-5 (I think it replaces the 20 in the Terk line of indoor antennass) over the internet because I was able to get it for $20 less than the TV-20 from CC and it looks a little "cooler" sitting on top of my center speaker. The built in amplifier (low-noise, variable up to 40 dB) may be overkill, but with it I can easily tune in to all the Bay Area digital broadcasts, so I'm happy with it.
 

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Ph.D.? Whoa, well, we need more of you around the fora, then! Great to have you.


Maybe you can help explain to me (in somewhat layman's terms---I'm a software engineer, and I don't understand RF at all) why the bowtie shape is so good, why two of them is better, and what the metal grate is for behind the double bowties.


Here's a close-up: http://ad1440.net/~kelly/closeant.jpg
 

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Discussion Starter · #18 ·
Hi Sean,


Thanks for posting the picture of your antenna. I assume from your picture you actually have yours outside. To answer your questions:


The two most important considerations in antenna design are the radiation pattern and the input impedance of the antenna.


The pattern is important for efficient use of your energy. For a satellite in space you want a very directive beam aimed at a particular spot on earth. You don’t want to waste your energy radiating into space. For a TV or radio transmitter, in general, you want to radiate energy everywhere in all directions in the horizontal plane, but not directly up in space. (Also, an antenna “behaves†similarly when it is transmitting or receiving, and often times, it is easier to visualize and describe things in terms of transmission instead of reception, which I will probably do as I write this).


Just as important to good antenna performance is the IMPEDANCE of the antenna. And impedance is usually the harder parameter to design for. To deliver maximum energy to a load, the impedance of the load must match the impedance of the transmission line. If you are using 300 Ohm twin lead for example, you want the impedance of your antenna to also be 300 Ohm. Any other impedance will result in energy reflected back due to the impedance mismatch. For example, connecting a 300 Ohm load to a 75 Ohm transmission line results in 36% of the energy being reflected, and only 64% utilized.


(One aside here… Twin lead wire has “balanced currents†on its conductors. Co-ax cables have unbalanced currents. The outer conductor is grounded and the inner conductor carries all the current. So the device that transforms a 300 Ohm twin lead to 75 Ohm coax does 2 things. It transforms from BAlanced current to UNBalanced current, hence the name balun, and it transforms the impedance from 300 Ohms to 75 Ohms).

Edit: Evidently, these baluns don't usually bother to match the impedance, so there's a significant insertion loss when using these.



Anyway back to subject matter, if you are operating at a single frequency (or extremely narrow bandwidth), there are many simple techniques for matching the impedance. But if you are operating over a wide bandwidth, such as the entire VHF to UHF TV band, that is not so easy.


UHF TV broadcast is horizontally polarized. The simplest antenna to use would be a thin wire dipole oriented horizontal, or parallel to the earth. Thin wire dipoles are however very narrow band from an impedance consideration and are generally used at their “resonant†length of slightly below half wavelength (about .47 wavelength). But the UHF TV band is almost an octave in bandwidth (470 MHz to 806 MHz). So if you use a thin wire dipole, it would work great at whatever frequency the dipole length is half a wavelength of, but get too far from that frequency, and your impedance mismatch losses will kill you: you would not be able to receive or transmit energy. But you can increase the bandwidth of your dipole by making it “fatter†than a thin wire dipole, and by the proper shaping of it… such as using a ..... bowtie shape. So a bowtie antenna is just a fancy dipole, and the particular bowtie with fin shape used in the RS antenna was probably done to achieve an impedance that is flat (or at least easily matchable) over the entire UHF band.


Without going into all the subtleties of array theory, 2 identical antennas connected in parallel, equidistant and equiphased from your source would result in double the power received. (As a quick side note, when you connect two identical antennas up in parallel, the net terminal impedance is split in two just as in regular circuits. Additionally, two antennas close to each other have a mutual impedance effect, i.e. the impedance of an antenna can be changed by having another antenna close to it, and all that has to be taken into consideration in designing your antenna system.)


With regards to the metal grate behind the antennas, it is simply a reflecting ground plane: essentially a mirror behind your antenna. The shortest UHF wavelength is 14 inches. As long as the grate spacing is small compared to your wavelength, it will appear more or less like a solid conductor – like bouncing a basketball off a chainlink fence.. (It’s cheaper and lighter for a grating instead of solid piece of metal). In theory, with the right spacing of the antenna off a sufficiently large ground plane, you can conceivably double the received power in one direction, at the expense of the other direction.


While we’re at it… let’s talk a little bit about gain and directivity. Both those terms are generally expressed quantitatively in dB. In general, when you are talking about dB’s, you are talking about a ratio of one quantity to another. In many cases, there is some standard reference quantity that is never mentioned. For antennas, it is usually an ideal isotropic antenna (one that radiates equally in all directions of 3 dimensional space). For the sake of describing it easier, imagine a dipole as the axis of earth. A dipole does not radiate (or receive) along its axis, i.e. in the directions of the north and south poles. If you fed the same amount of power to a matched isotropic antenna and a matched dipole, the dipole will transmit more power along the equator compared to the isotropic antenna (conservation of power… if no power is going to the poles, more has to go to other directions). The amount of maximum power ratio COMPARE TO AN ISOPTROPIC ANTENNA is known as the directivity (or directive gain) or an antenna. For a short dipole, the directivity is 1.5 (or 1.76 dB) and for a half-wave dipole, it is 1.64 (or 2.15 dB). You sometimes see it written in dB’s as dBi to explicitly indicate that the reference is an isotropic antenna, but most times it is just written as dB’s with the reference to isotropic implicit.


What GAIN is, then, is simply DIRECTIVITY with all losses taken into account, such as resistive losses, impedance mismatch losses, polarization losses, etc. i.e. real world considerations. So for the RS double bowtie antenna: a single bowtie would have a directivity of approximately 2 dB, rounding to the nearest integer. (I think most people are aware that dB is defined as 10*log(power ratio) and twice the power is a 3 dB increase and 10 times the power is a 10 dB increase). Using 2 dipoles, you would increase your maximum directivity by 3 dB, and having a ground plane increases by a maximum of another 3 dB. So your directivity would be at best 8 dB. But across the entire UHF band, you will have impedance mismatch losses, also the metal grating is not a perfect ground plane, and is finite in size, so you wouldn’t have exact 3 dB due to your ground plane. If the match is decent over the entire band, you might end up losses of 1 to 3 dB, so your antenna would probably have a net GAIN of 5 to 7 dB depending on where you are on the UHF band. This is a quantity of how much better you are doing in the direction of maximum radiation (and reception) compared to using an isotropic antenna. And if you look up the specs of this antenna on Radio Shack’s website , that’s exactly what they spec this antenna to: “Approx. 5 ~ 7 dB â€


Sorry this is so long, but I hope it’s clear and some people find this information useful.
 

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Quote:
Some folks have modified the bowtie and removed the 300 ohm lead and replaced it with a 75 ohm coupler right at the connectors on the antenna. Reports indicated a vast inprovement in signal quality and level.
Does anyone know the P/N of this coupler?

Also, how does the D-Bowtie fare against the elements when placed outside? Thanks in advance. :)
 

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Quote:
Originally posted by Frank_S



Does anyone know the P/N of this coupler?

Also, how does the D-Bowtie fare against the elements when placed outside? Thanks in advance. :)
RS #15-1140, $2.99


no... i'm not affiliated w/ RS. :)
 
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