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Discussion Starter · #1 ·
Similar to my other thread, but I'd like to keep this one a bit more focused:


I have a Radio Shack twin lead double bow-tie antenna and I'm afraid I have an impedance mismatch. I have both types of impedance transformers from radio shack (the little box with an F-type (female) and the two screw terminals;; and the inline type with a male f-type and twin leads on the other end).


I've tried both on the end of the wire with little result. I still get a lot of signal from the transmission line and not the antenna (the signal varies immensely, depending on the position of the wire).


Which one should I use, or are they both wrong? Should I take off the 300 Ohm lead and just put the transformer directly on the antenna?


Thanks for any advice,

Anthony
 

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The antenna has a 300 Ohm impedance and is matched to the twin lead which is also 300 Ohm impedance. The transformers you describe are 300 to 75 Ohm matching transformers. Your TV has a 75 Ohm input. I would say use the first one you described, but either one will work.
 

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Discussion Starter · #3 ·
But does that explain why the 300 Ohm twin lead seems to be getting more signal than the antenna? If the lead drapes straight down, I get one signal, move it to the left an inch (without changing the antenna) and the signal goes up. Move it a little more, and it goes down.


I'm almost tempted to get a spool of 300 Ohm wire and run it around my apartment. http://www.avsforum.com/ubb/smile.gif


One more question: is there any difference between the two transformers I described? They seem to do the same thing, but the inline one seems to be of much better build quality.


Thanks for the help,

Anthony
 

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Quote:
Originally posted by Anthony:

I'm almost tempted to get a spool of 300 Ohm wire and run it around my apartment. http://www.avsforum.com/ubb/smile.gif
I believe that this would be a big mistake. My understanding is that 300 ohm wiring is an excellent antenna unto itself. This could introduce ghosting or other undesirable effects.



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Anthony, I really think you've either got a bad antenna or a poor connection from the twin lead to the antenna or balun. The best bet would be to connect the transformer (the one with the 2 leads) directly to the antenna terminals and run good coax down to the set. I would keep the coax much less than 50' unless you want to buy a preamp.


300 Ohm twin lead is really yesterday's technology. It actually has less loss when brand new (name brand stuff)but deteriorates very rapidly. It also is sensitive to position, it can't be near any metallic object, should be at least 6" away, and thus you will have to use a lot of ugly standoff insulators.


Make sure your antenna is in good shape and making good connection to the transformer, and use good quality RG-6 between it and the set.


Bob Smith
 

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Anthony,


Without the catalog numbers I'm unsure which RatShack baluns you have. However, to minimize induced interference (which is why 300 ohm twinlead is largely out of favor despite its superior signal loss characteristics) use one of the baluns that have ~6" of twinlead at one end and a female "F" connector at the other. Use 75 ohm coax from the balun to the receiver/TV and connect the balun's twinlead segment directly to the antenna. 75 ohm coax is essentially immune to induced interference. RatShack has something suitable in their 15-1140 ($2.99) and 15-1143 ($3.99). Channel Master has a heavier duty model available at Lowes for ~$5.00. Rat Shack also has a gold plated model for $3.99 (15-1230), but, my own opinion is that plating helps RatShack more than it helps the end user. (Least ways I've never been able to see an on-screen distinction...)
 

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Discussion Starter · #7 ·
Thanks for the input guys.


I was kidding about the 300 Ohm around my apt. I think that's the problem. It's probably picking up interference from everything on my rack and the rebar in the wall (my apartment is very old, concrete all around with some plaster covering).


I'll try mounting the balun straight to the antenna; unfortunately, it doesn't have nice wing-nuts like the CM, so I'll actually have to cut the wire off and solder the leads to the balun on directly. I have an extra one lying around. I'll have to check the part number, though.


Thanks again,

Anthony
 

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I have been experimenting with the same thing.


I have a DTC-100, connected to a 25ft coaxial cable, then to my Radio Shack Double Bowtie antenna. I have tried several 300/75 ohm adapters, with varying luck.


The more complex splitter/combiner units were completely useless. I got almost no signal (as seen in the signal strength meter on the DTC-100).


I have had the most success with the simple F connector to two post 300 Ohm connector, along with a female/female coaxial "gender changer".


Recently, I found a simple adapter with the two post 300 Ohm connector, and both a male & female coaxial connector. I thought I might gain something by eliminating the gender changer. But, when hooking it to the female connector, I got terrible signal. When I put the gender changer in, and hooked it to the male connector, I got a strong signal.


So, even though all of the inputs were labeled in/out, some appeared to be passing no signal. My simple solution is working, but I am not sure that it is the best I can do.
 

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Discussion Starter · #9 ·
Well, last night I removed all of the 300 Ohm lead from my bow-tie antenna and soldered an inline transformer directly to the antenna. Then it was all coax from antenna to DTC100. The difference is measureable and good. The antenna is less "picky" and signal strengths are up about 10 on all channels. I think the twin-lead was picking up a lot of interference from everything and that was degrading signal quality.


By this theory, the 300 Ohm twin lead wasn't acting as an antenna, but destroying signal quality. Some positions, it was fine, in others it elminated the signal.


So basically, this modification extended the shielding properties of 75 Ohm coax all the way to the antenna. So far it's working.


Thanks for the input,

Anthony
 

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Quote:
Originally posted by Anthony:

By this theory, the 300 Ohm twin lead wasn't acting as an antenna, but destroying signal quality.
It was acting as an antenna. Just not for the signals you wanted. http://www.avsforum.com/ubb/smile.gif

Glad things are working for you.



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I have been thinking about doing the same thing as Anthony, or at least shortening the length of the 300 Ohm cable to

minimize the problems.


Anyone have thoughts on attaching an adapter to the antenna?

I had ruled out soldering mine, because I didn't think it would be a strong enough attachment.


The 300 Ohm cable is riveted into the antenna. I could remove the rivets and attach the adapter using a small bolt/nut. Or, I could just drill a new hole in it & do the same.


Are there any special requirements for attaching this thing? Are steel bolts ok?
 

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Discussion Starter · #12 ·
Sorry, I was a bit misleading. I didn't solder directly to the frame, I cut the leads to about an inch and then spliced the wire from the isolation transformer in. Worded another way, I used the rivets and about 1" of 300 Ohm cable already attached.


If this is confusing, let me know and I'll try to post an illustration.


Good luck,

Anthony
 

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Congratulations, Anthony! I had no idea the RatShack double bow-tie UHF antenna used a captive length of 300 ohm twinlead. That bit of engineering did complicate your task, but, fortunately your expertise in the manly art of soldering served you well. You've essentially fixed a RatShack design flaw permanently. One thing you might additionally look into would be to attach a nylon cable tie to the rear screen and bind the body of the balun in place. Leave a bit of slack in the twinlead to elliminate any chance of your solder connections being pulled apart.
 

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I finally got around to changing my adapter this evening...

I trimmed off all but 5-6" of the 300 Ohm wire attached to the antenna. I then attached one of those 300-75 Ohm cylindrical adapters.


Some channels, like KPIX and KQED, have always been strong for me. Others, like KBHK and KICU required me to redirect my antenna. And some, like KRON, were sporadic.


In my limited testing (one evening), it looks like it makes a noticeable difference. All of the channels have been strong (most in the 70-80 range), and I haven't seen a dropout yet.
 

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Anthony and tji,


The results both of you have been getting still seem really marginal. Are your antennas indoors or on a rooftop? tji, you should have no problem getting clear signals in Mountain View unless you are using an indoor antenna and are having problems with reflections from rebar, foil backed insulation or other items like this.


..Doyle in Sunnyvale
 

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Discussion Starter · #16 ·
Indoor. I live in a first floor apartment with no balcony. It's pre WWII construction (parts of the complex are pre WWI). Mostly concrete, brick, and plaster (which leads me to believe there's a lot of rebar around).


Anyway, things seem to be working now. My next place will either be a house, or a townhouse with a balcony, porch, or roof access.


Anthony
 

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I am also using an indoor antenna (Radio Shack double bowtie). I get good reception here in Mountain View, but not great. I have had to tweak the position of the antenna to get channels with no breakup. Since removing the 300 Ohm cable, I can get all channels in the area without moving the antenna. I get a very occasional signal dropout, but overall it's pretty good.


I live in an apartment, but I am on the second floor, with a balcony. I recently put up a DirecTV dish, on a mast mounted in a tripod on my balcony. I may mount a small outdoor UHF antenna on that mast. But, since my current antenna is working very well, I may not need to do that.
 
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