How to build a UHF antenna... - Page 66 - AVS Forum | Home Theater Discussions And Reviews
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post #1951 of 4912 Old 03-03-2009, 02:24 PM
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Quote:
Originally Posted by nepaeric View Post

I had old, 2-wire, 14-2 laying around from when we re-wired our house, and 300ohm would not let me get off without stripping it down and building ladder wire out of small strips of vinyl siding. (He was quite forceful and aggressive in doing so, almost to the point of scaring me.) Others thought it might be a waste of time, but I found out otherwise. - Trust those who have done this before. (They know what they are talking about.)

In any case, go and follow the advice of 300 ohm and build yourself some homemade, super, ultra, low-loss 14 guage, 300 ohm ladder wire. You'll be glad you did.

Warm Regards,
Eric


For others is a good move to plan out the job in detail before purchasing wire and making the ladder line.

Routing and support and penetration into building are all tricky details that are best worked out first for making it an easy well performing job.
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post #1952 of 4912 Old 03-03-2009, 03:50 PM
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I'm suspicious.

I can understand how perfect ladder wire can be considerably better when suspended in the air, but how can it be better in real-world installations?

The distance between the wires varies constantly over its length (impossible to keep it perfectly spaced) and bare wire randomly touches building materials/supports all the way, so the dielectric varies constantly over its length too. This must wreak havoc with the impedance, creating mismatches all the way.

Gary
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post #1953 of 4912 Old 03-03-2009, 04:50 PM
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Trust those who have done this before. (They know what they are talking about.)

But I didnt build my own 300 ohm open wire, I bought it at Layfette Radio many years ago, heh. IIRC, it was about four times the price of regular 300 ohm wire at the time.

Quote:


(impossible to keep it perfectly spaced)

No its not, and his 14 gauge is easier to keep perfectly spaced than my 18 gauge commercial stuff.

Quote:


bare wire randomly touches building materials/supports all the way, so the dielectric varies constantly over its length too.

Thats where you use open wire standoffs and keep it away from any close by metal by at least about a foot. My commercial ones were about a foot long with another 2 inches of plastic.

Heres the link: http://www.qsl.net/co8tw/openline.htm

Quote:


For others is a good move to plan out the job in detail before purchasing wire and making the ladder line.

Definately.
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post #1954 of 4912 Old 03-03-2009, 05:25 PM
 
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Quote:
Originally Posted by nepaeric View Post

What a difference it made! I have no idea what the loss ratio per foot is, but it's got to be exceptionally low by comparison to RG-6 and even RG-11.

There is a huge difference.

The following numbers for transmission line attenuation are based on a 100' length, in center of the UHF TV band.

When the SWR = 1:1 (i.e. matched):

The loss in open wire transmission line (ladder type) is .53 dB

The loss in open wire transmission line (window type) line is .81 dB

The loss in RG6A coax (Belden 8215) is 7.39 dB


When the SWR = 5:1 (e.g. typical across UHF TV band frequency range):

The additional loss in open wire transmission line (ladder type) is +.69 dB.

The additional loss in open wire transmission line (window type) is +.97 dB

The additional loss in RG6A coax (Belden 8215) is +2.49 dB


Total losses (e.g. matched + SWR):

Open ladder line = 1.22 dB

Open window line = 1.78 dB

RG6A coax = 9.88 dB

Notes:

The amount of signal that is lost, corresponding to the added 8.66 dB loss difference between ladder line vs coax) is 86%.

Another advantage of the parallel transmission lines, is that they are balanced and not susceptible to acting as part of the antenna (e.g. preserves the antenna pattern directionality, less multi-path etc.). However, installation does require more care (no sharp bends, placement close to metal objects etc.).
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post #1955 of 4912 Old 03-03-2009, 06:35 PM
 
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Quote:
Originally Posted by nepaeric View Post


I am also inclined to believe that it affords less resistance in the signal from the antenna to the TV's tuner by putting the balun last in the circuit, but that's just my theory, I surmise, and could absolutely be wrong on this.

This is particularly true when using a RG6 coax.

You want only the signal picked up by the directional pattern of the antenna to be delivered to the receiver.

When using parallel transmission line, the currents from any extraneous electric fields picked up, automatically cancel out as the conductors will have equal and opposite currents. However, when using coax, the outside surface of the shield (e.g. skin effect) become part of the antenna and there is no longer an equal and opposite current to provide cancellation.

dipole.JPG

This is what your antenna really looks like.

dipole2.jpg

As a result, more than likely, the quality of signal will be impaired (e.g. pattern distortion, added multi-path etc).

The solution is simple.

Put a 1:1 current (or choke) balun where the coax connects to your TV. (not at the antenna).

This will set up a high impedance path on the outside of the shield and isolate the transmission line (from becoming part of the antenna).

A simple and effective example of a 1:1 choke balun is realized by placing suitable ferrite material at the end of the coax feed or winding the coax into a coil.

Here is an example.

http://www.ham-radio.com/k6sti/balun.htm
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post #1956 of 4912 Old 03-03-2009, 07:08 PM
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Heres a picture of an old standoff I have :



I also had others where the plastic piece was even longer and there was an additional hole in it for the round type of twinlead, IIRC.

Quote:


The loss in RG6A coax (Belden 8215) is 7.39 dB

That must be the old hard plastic center type. The newer common foam core stuff is mostly quoted at 5.5 db per hundred foot, hence 1 db loss every 18 ft.
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post #1957 of 4912 Old 03-03-2009, 08:09 PM
 
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Quote:
Originally Posted by 300ohm View Post

The newer common foam core stuff is mostly quoted at 5.5 db per hundred foot, hence 1 db loss every 18 ft.

Actually that is at the highest frequency...The most common RG6 specs per 100ft are as follows:

1.6 dB @ 50 MHZ

2.6 dB @ 170 MHz

4.6 dB @ 450 MHz
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post #1958 of 4912 Old 03-03-2009, 08:49 PM
 
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Quote:
Originally Posted by 300ohm View Post

That must be the old hard plastic center type. The newer common foam core stuff is mostly quoted at 5.5 db per hundred foot, hence 1 db loss every 18 ft.

The loss figures I used were all from a graph in an antenna hand book where the only RG6 was Belden 8215, which was 7.39 dB @ 600MHz (mid UHF TV band).

Just checked their catalog.

http://www.belden.com/pdfs/03Belden_...ial_Cables.pdf

Their 8215 does have a solid polyethylene insulation and has a loss of 8.1 dB / 100' @ 700 MHz.

Their 9290 has foam polyethylene insulation and has .9 dB less loss, 7.2 dB / 100' @ 700 Mhz.

Their 89248 uses flurorinated foam insulation, at 6.1 dB / 100'. They also have 9248 which uses a gas-injected foam at, 5.3 dB / 100'.

Then there is always the coax made in China.
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post #1959 of 4912 Old 03-03-2009, 10:16 PM
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Right now, my Kodak "Easy Share" digital camera is on the blink, and isn't making it very easy for me to share photos. I will either figure out why it froze, or pitch it in the trash and buy a new fangled one that perhaps will work for this task. As soon as I do, I will send you some photos, IDRick, and thanks a lot for offering to post them. I've got some other photos, though, I will send along, so feel free to share those, if you like.

Then, when we get some warmer weather, (like it was a few days ago, when I set this all up) I will get some photos up on the roof, where it attaches to the antenna. Right now, it's freezing up there!

I used crimp-on eyelet connectors at the antenna feed point location, and at the other end, where it attaches to the push-on type balun, for a secure connection, and somewhat, semi-professional installation appearance that looks stealth, and is more than somewhat functional. I used short pieces of 1/2" CPVC, cut down, along with strips of vinyl siding material, and secured the 14 guage to it with a dab of goop.

It may appear to be what I might refer to as a combination of validated technical research, along with a bit of "sheeny" engineering and good, old-fashioned, down home, barefoot, toothless, red-necked, hillbilly ingenuity approach, but who can argue with results. Either way, my super, ultra, low-loss 300 ohm ladder wire is 100% Made In The USA, with pride stuff, (assisted by a matching transformer balun, Made in Japan), and virtually none of it made in China, Singapore, Taiwan, Bangladesh, or Malasyia This stuff is all copper, vinyl and plastic, but worth it's weight in gold. I highly recommend you all try this.

It is not as hard to maintain a consistant distance as one might think. The plastic material holds it the proper distance apart quite well. The 14 guage is rigid, and retains it's straightness quite well. Where you have to be careful is where you bend it, not to use sharp bends, but instead, slightly curl it in the direction of the bend to maintain this spacing. Agreed, it might likely exhibit non-linearities if you didn't maintain this distance apart, and undoubtedly requires patience in careful routing out the wall and to the antenna. Should those lines cross at any point or come closer together, it would surely kill the design.

300ohm, thanks for the best advice to do this. It was a lot of work, but rewarding in the end. Incidentally, what distance apart is your 18 guage? If I were to get a little more inventive, about how far apart would it need to be for 12 guage? More than 2"?

Copper wire is really cheap right now compared to a few months ago, at these low prices of 10-15 cents per foot. Buy it at a good, reputable electrical supplier.- not a home center.- Lowe's and Home Depot are out in space, and still selling this stuff at old market pricing. It would be cheaper than RG-6 to do. I hope at least one of you tries this shrewd technique and timeless technological marvel this to confirm my results, and post your findings here, as well.

Many thanks again, 300ohm.

Best Regards,
Eric
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post #1960 of 4912 Old 03-04-2009, 01:10 AM
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Quote:
Originally Posted by 300ohm View Post

No its not [impossible to keep it perfectly spaced], and his 14 gauge is easier to keep perfectly spaced than my 18 gauge commercial stuff...

Heres the link: http://www.qsl.net/co8tw/openline.htm

Interesting read at the link above. However, Cebic states:

"From the table of values, it is clear that constructing a 300-Ohm line in open-wire fashion would be difficult. First, the spacing is narrow and may require more spacers to keep the wires aligned. Second,

small changes of spacing will create larger changes of impedance than with wider-spaced lines". Sure, you could go for 600ohm lines, but then you'd have to construct an unusual balun to match the 600ohms to 75ohms at the other end.

Quote:
Originally Posted by 300ohm View Post

Thats where you use open wire standoffs and keep it away from any close by metal by at least about a foot. My commercial ones were about a foot long with another 2 inches of plastic.

OK, I understand the standoff concept. That makes good sense. As Cebic says, "Nothing ruins the performance of beautifully made parallel transmission line like careless installation".

Gary
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post #1961 of 4912 Old 03-04-2009, 02:06 PM
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Well yeah, nobodys saying that its easy, heh.
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post #1962 of 4912 Old 03-04-2009, 02:55 PM
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FYI: An on-line loss calculator for many different brand name coax cables and balanced ladder lines:
http://www.ocarc.ca/coax.htm

Includes wet and dry loss for watchman balanced ladder lines:
http://www.thewireman.com/products.html

I found some Belden twin-lead specs:
Belden 9085 twin-lead:
1.4 dB/100' at 100 MHz
5.9 dB/100' at 1000 MHz

Belden 8225 twin-lead:
1.1 dB/100' at 100 MHz
4.8 dB/100' at 1000 MHz

You can also do a search for fol. specs and see detailed construction:
http://www.provowire.com/results.asp

Provowire 0550 is "equivalent" to Belden 9085:
1.4 dB/100' at 100 MHz
2.8 dB/100' at 300 MHz
3.8 dB/100' at 500 MHz
4.8 dB/100' at 700 MHz

Provowire 0790 is "equivalent" to Belden 8225:
1.04 dB/100' at 100 MHz
2.5 dB/100' at 300 MHz
3.5 dB/100' at 500 MHz
4.5 dB/100' at 900 MHz

So you're probably better off using RG-6 coax vice typical plastic&foam twinlead....

==================================================
EDIT (9Jul2009): Loss data for Channel Master 9354 (and 3055) twinlead:
https://www.avsforum.com/avs-vb/showt...php?p=16796125

Channel Master Model 3055, 300 Ohm Twin-lead (3055 and 9354 is the same,
3055 is various hank lengths and 9354 is 500 ft. spool). The wire is 20 gauge,
7 strand pure copper.

dB Loss per 100 Feet:

MHz-- 57 85 177 213 500 650 800 900
Dry-- 0.9 1.2 1.5 2.0 3.2 3.8 4.5 5.4
Wet-- 1.3 1.7 2.3 3.2 7.2 9.5 11.1 13.5

Lowest loss appears to be Belden 8225 and Provowire 0790....

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post #1963 of 4912 Old 03-04-2009, 03:47 PM
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Quote:


Incidentally, what distance apart is your 18 guage?

Its .55 inches center of wire to center of wire. Which according to the chart in that link would put it at around 380 ohms, so I dont know if I got ripped off, heh.
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post #1964 of 4912 Old 03-04-2009, 04:06 PM
 
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Quote:
Originally Posted by 300ohm View Post

Its .55 inches center of wire to center of wire. Which according to the chart in that link would put it at around 380 ohms, so I dont know if I got ripped off, heh.


How could that be (you got more ohms for your dollar)?


Here are some neat JAVA calculator/modeling applets that cover the theory, design and application of transmission lines.

Two wire transmission line calculator:

http://www.amanogawa.com/archive/TwoWire/TwoWire.html

Coaxial transmission line calculator:

http://www.amanogawa.com/archive/Coaxial/Coaxial.html

Transmission line impedance calculator:

http://www.amanogawa.com/archive/Lin...Impedance.html

Instructional tutorials in .pfd format are here:

http://www.amanogawa.com/archive/transmissionpdf.html

Lots more applets here (e.g. SWR, matching, smith charts, broadband transformer etc.):

http://www.amanogawa.com/archive/transmissionA.html
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post #1965 of 4912 Old 03-04-2009, 04:44 PM
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Quote:
Originally Posted by GSB View Post

Indeed, dynamic multi-path seems to be the killer. Somehow, an ATSC (digital) tuner manages to lock exceedingly well on static multi-path, even when its as bad as mine!

Gary

I have had the same experience. I was able to reduce my static multipath problem by adjusting the aim of my antenna using the signal quality bar (which indicates BER) of the Apex DT502 CECB, even after peaking the strength with my SLM.

I also have dynamic multipath reflections caused by cars on a well-traveled street in front of my CM4221 antenna. My weakest channel still suffers an occasional break-up, but the rest maintain lock because the ability of the FEC to make corrections is not exceeded. The FEC has more work to do with the weak signal that has a poor SNR because it is closer to the noise floor.

This demonstrates the need for the new ATSC M/H standard which adds training signals to the transmitted signal. http://www.tvtechnology.com/article/61736
http://www.tvtechnology.com/article/16632
https://www.avsforum.com/avs-vb/showthread.php?t=1029236

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post #1966 of 4912 Old 03-04-2009, 05:13 PM
 
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Quote:
Originally Posted by rabbit73 View Post

This demonstrates the need for the new ATSC M/H standard which adds training signals to the transmitted signal. http://www.tvtechnology.com/article/61736
http://www.tvtechnology.com/article/16632
https://www.avsforum.com/avs-vb/showthread.php?t=1029236

Glad that the ATSC modulator our company just designed for a company that designs and manufactures DTV broadcast equipment was implemented using a couple of large Field Programmable Gate Arrays (FPGA).
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post #1967 of 4912 Old 03-04-2009, 06:44 PM
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Some really fascinating and valuable stuff posted here, guys. Thanks for sharing it. I'm hooked!

Gary
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post #1968 of 4912 Old 03-04-2009, 06:54 PM
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Quote:
Originally Posted by holl_ands View Post

So you're probably better off using RG-6 coax vice typical plastic&foam twinlead....

High quality coax has come a long way. I'm pretty much sold on using coax now... there's just one more burning question... How bad is the loss when connecting a balun at the antenna vs. at the TV?

Gary
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post #1969 of 4912 Old 03-04-2009, 08:23 PM
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Quote:


How could that be (you got more ohms for your dollar)?

Hmm, it did come in a 300ohm bag. Shouldnt they have used a bigger bag ???

On that first calculator link, inputting my data, it comes up with 242 ohms. So, 380 ohms from Cebiks site and 242 ohms from that other site average to 311 ohms. Close enough, heh.


Quote:


High quality coax has come a long way. I'm pretty much sold on using coax now... there's just one more burning question... How bad is the loss when connecting a balun at the antenna vs. at the TV?

I think serndipity was referring to connecting the balun at the TV using 300ohm downlead, and it makes sense that there would be less loss and other benefits.
Directly connecting the balanced antenna to the unbalanced coax and then connecting the unbalanced coax to the balancing balun seems like there would be a lot more loss.
Coax is good to use, just get a low noise preamp to take care of the losses after the antenna terminals on longer runs.
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post #1970 of 4912 Old 03-05-2009, 09:09 AM
 
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Quote:
Originally Posted by GSB View Post

High quality coax has come a long way. I'm pretty much sold on using coax now... there's just one more burning question... How bad is the loss when connecting a balun at the antenna vs. at the TV?Gary

The simple answer is that when using coax, both ends of the transmission line.

To minimize SWR losses, at the antenna, a 4:1 step down is required to match the 300 ohm antenna to the 75 ohm coax.

Then, to isolate the coax from becoming part of the antenna, a 1:1 current balun required at the TV.

Fortunately, this is can be simply realized by winding the coax into a coil or adding ferrite material.

See post 1957:

https://www.avsforum.com/avs-vb/showt...798265&page=66

When using 300 ohm balanced transmission line, only a 4:1 current balun at the TV is needed (because the TV has a 75 ohm unbalanced input).
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post #1971 of 4912 Old 03-05-2009, 02:11 PM
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Ken Nist (who runs hdtvprimer) wasn't impressed with DIY 4-Bays either:
http://www.hdtvprimer.com/ANTENNAS/cm4221.html

PS: He also analyzed new DB-8 and CM-4228HD 8-Bay Antennas, finding problems (mostly in the DB-8):
http://www.hdtvprimer.com/ANTENNAS/TemporaryPage.html
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post #1972 of 4912 Old 03-05-2009, 03:52 PM
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Quote:


Ken Nist (who runs hdtvprimer) wasn't impressed with DIY 4-Bays either:

He obviously is referring to the u-tube disasters, and not mclapps models (or my Baltimore Bowtie)
heh. Those designs are superior to the CM4221HD for their intended purposes.

Im glad to see he updated his site.
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post #1973 of 4912 Old 03-05-2009, 03:58 PM
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Quote:
Originally Posted by 300ohm View Post

I think serndipity was referring to connecting the balun at the TV using 300ohm downlead, and it makes sense that there would be less loss and other benefits.
Directly connecting the balanced antenna to the unbalanced coax and then connecting the unbalanced coax to the balancing balun seems like there would be a lot more loss.

Thanks for the response. I do understand this. I didn't state my question very well. What I meant to ask is:

We have explored the losses in coax vs twin-lead, but how much worse are the balun-induced losses...
  1. when using a coax run, with the 4:1 balun at the antenna end, vs.
  2. using a 300ohm twin-lead run, with the 4:1 balun at the TV end?

Gary
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post #1974 of 4912 Old 03-05-2009, 04:08 PM
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Quote:
Originally Posted by serndipity View Post

The simple answer is that when using coax, both ends of the transmission line.

??

Quote:
Originally Posted by serndipity View Post

To minimize SWR losses, at the antenna, a 4:1 step down is required to match the 300 ohm antenna to the 75 ohm coax.

Then, to isolate the coax from becoming part of the antenna, a 1:1 current balun required at the TV.

I thought that a typical 4:1 voltage balun takes care of both the 4:1 step-down and the balanced-to-unbalanced isolation.

Quote:
Originally Posted by serndipity View Post

Fortunately, this is can be simply realized by winding the coax into a coil or adding ferrite material.

See post 1957:

Unfortunately, the coax coil is only good for a narrow band of frequencies. The example in that link is good for the FM band only. Also, the coil-size required for UHF bands would be so small as to seriously violate the coax bend radius.

Gary
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post #1975 of 4912 Old 03-05-2009, 04:46 PM
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Then, to isolate the coax from becoming part of the antenna, a 1:1 current balun required at the TV.

A lot of the circa 1980s sets had that built in from the 75ohm connection on the back on the way to the tuner. The 1998 29 inch Panasonic set Im now in the process of scraping doesnt, but the European style connector is directly on the tuner, which is on the back of the pcb of the set.
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post #1976 of 4912 Old 03-05-2009, 04:46 PM
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Quote:
Originally Posted by holl_ands View Post

Ken Nist (who runs hdtvprimer) wasn't impressed with DIY 4-Bays either:
http://www.hdtvprimer.com/ANTENNAS/cm4221.html

He (Ken Nist) doesn't mention the DIY versions in your link... Is there an incorrect or missing link?
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post #1977 of 4912 Old 03-05-2009, 04:52 PM
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Originally Posted by GSB View Post

Thanks for the response. I do understand this. I didn't state my question very well. What I meant to ask is:

We have explored the losses in coax vs twin-lead, but how much worse are the balun-induced losses...
  1. when using a coax run, with the 4:1 balun at the antenna end, vs.
  2. using a 300ohm twin-lead run, with the 4:1 balun at the TV end?

Gary

Thats what were saying, the 300ohm twin-lead run, correctly installed, with the 4:1 balun at the TV end, when its DRY (important consideration) is significantly lower loss.

When its wet, its another matter. But then again, maybe rain gets into the outdoor balun or the coax, which increases the losses on it.

Quote:


when using a coax run, with the 4:1 balun at the antenna end, vs.
using a 300ohm twin-lead run, with the 4:1 balun at the TV end?

In particular answer to that, everything else being equal, the indoor balun will be drier, and more closely operating to design temperature, than the outdoor balun.
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post #1978 of 4912 Old 03-05-2009, 05:23 PM
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He (Ken Nist) doesn't mention the DIY versions in your link... Is there an incorrect or missing link?

???? I saw it. Its a footnote at the end.
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post #1979 of 4912 Old 03-05-2009, 05:42 PM
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Quote:
Originally Posted by holl_ands View Post

Ken Nist (who runs hdtvprimer) wasn't impressed with DIY 4-Bays either:
http://www.hdtvprimer.com/ANTENNAS/cm4221.html

PS: He also analyzed new DB-8 and CM-4228HD 8-Bay Antennas, finding problems (mostly in the DB-8):
http://www.hdtvprimer.com/ANTENNAS/TemporaryPage.html

Looks like according to his computer simulations nobody is doing anything right
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post #1980 of 4912 Old 03-05-2009, 05:45 PM
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Ooops, my bad. I skimmed looking for DIY. Was rather obvious section with a box around it...
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