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Old 13th April 2009, 01:18 AM   #1
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Default Antenna woes, I need guidance

Hi All,

I live in an 83 year old house that has METAL siding (way before vinyl existed). And even though I live directly in the middle of a city of about a million people, reception hasn’t always been optimum even though the towers are relatively close and have ample transmitting power. I thought the metal siding was a large part of the cause, but it turns out not to be.

Since I listen to the radio a lot, I have wanted to build some sort of nice FM antenna for the longest time. I like omni’s as you don’t have to deal with rotating the darn thing and since I live in the middle of everything, I should be able to pick up things nicely if the omni is made well, nice cable is used and its positioned high (and better yet, outside). So I started on my research to do such a project. There are a couple ‘weird’ things about what I have experiencee though, which is part of the reason I went on this quest.

The tuner in the house is a decent Awai shelf system. It has two push in terminals for the FM antenna and it is labeled 75 ohm on the back. This confused me because I thought most of those push in terminals were to put in stripped wires of 300 ohm wire in?! I thought most 75 ohm antenna terminals on stereos used F connectors? Maybe I am crazy but it almost seemed like an error that they put a 75 ohm label on the back by the terminals…lol.

So, I stripped a short piece of RG6 separating ˝ inch of the center conductor and ˝ inch of the shielding and inserted that in the two terminals. It was basically a pig tail because I put an F connector on the end of that and plugged in whatever 75 ohm antenna I wanted to. Well, that was the problem: The reception was absolutely HORRIBLE when I did that. In fact, I got BETTER reception WITHOUT ANY wires going into the antenna terminals than with any kind of antenna I tried. I couldn’t understand it. At this point I knew my bad reception was not caused by the metal siding because the reception was nearly perfect even with no antenna used!

Well, then I got an HD tuner and said: “self, its time to build that FM antenna you always wanted to build” So for my first DIY antenna, I got a piece of RG-6 and stripped 2 feet, 4.625 inches of the shield off completely exposing the center conductor. I cut the remaining shield off at about 1.5 inches in length, twisted it in a drill chuck, and crimped a piece of copper wire to it (same AWG as the center conductor) and also made it the same length as the center conductor. So both were fixed at 98.1 MHz. A simple dipole mounted vertically as an omni which should work well.

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|_____________RG-6______42’___________________> Tuner
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In my research I had seen where you are supposed to put one of those ferrite doughnut things and loop the RG-6 three to five times near the base of the antenna. Reference this page:

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=15311

I followed their advice even though it looks to me like it is under the minimum bending circumference for RG-6. The antenna was then mounted (as shown below) way outside and up higher and no longer in a metal house. The thing has to be at least 30 feet off the ground AND (lucky for me) I am at the pinnacle of a high spot of terrain. The conditions really couldn’t be better. It runs with 42 feet of RG-6 into the room notated. Now the frustration:

Both the Aiwa and the HD tuner are installed in that room. I hooked up the new antenna to both of the units and the results were awful! Stations that are close and powerful that ALWAYS come in were not coming in on my new antenna. I was confused and frustrated. Then I got a slap in the face: using no antenna at all on the Aiwa got better reception than using my new antenna that’s mounted outside and way up high. And using the cruddy white antenna (shown in last pic) that comes with receivers got me better reception than the new antenna even while it was inside a metal house and positioned at a lower elevation.

Can anyone with knowledge in this area maybe see where I went wrong by anything I’ve mentioned or the pictures below? Is the balun screwing me up that I got from the link above? Because then I found another balun design that is totally different and doesn’t even go around a ferrite doughnut. Reference this page:

http://ham-radio.com/k6sti/balun.htm

The one from the first link is very different from the second link. I doubt the 42 feet of RG-6 is causing the problem due to cable loss. I would think the extra signal strength would easily make up for it.

I really find antenna technology fascinating and fun (well, at least it was fun…lol) Now its just become a headache as it was a total failure. I was hoping to make my HD unit really shine with this setup. Any information you guys could provide would be truly appreciated and I’m sorry for such a long post.

John


(antenna is in the white pole hanging out from the balcony)

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Old 13th April 2009, 02:52 AM   #2
picopos is offline picopos  Portugal
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hey
i did antennas for ham radios long ago...
-omnidirectional-put the antenna in vertical axis(regular gain no more then 3 with 1/4 wave lenght ground plane)
-directional-put it in horizontal axis(easyly u get 9db gain)

in your case i think u would be better off making something with 2 telescopic antennas ,a base to hold them and to attach to the pole(vertical if u want it omni)
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Old 13th April 2009, 03:16 AM   #3
picopos is offline picopos  Portugal
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other things...
in radio receiving,the signal is so weak and susceptible to noise that cabling is very very important,try finding rg-58 good quality,keep the connections as short as possible,never have kind of extension connections,keep it sealed until antenna,the shield of the cable,let's call it ground,should always isolate the inner cable or else your antenna starts were the inner cable stops being shielded,that is to mantain the impedance of the line......when building a regular antenna like i'll try to post latter,the space beetwen the dipoles makes the impedance ,this is very important,has your signal wont flow to the receiver....

bigger is no better,the size has to match the desired frequency,in fm broadcast u find the center freq and u calculate it in size, (u have to find that,i'm not very sure of it now,but it's something like the freq times speed of sound and then latter multiplied by .95 that is the factor of the ionosphere effect on the radio waves).....then u make it full wave,that is the size of the wave(many times impractible)or half wave(half size) or 1/4 wave (mostly used) for Dx at least u should have half wave......

there's many things to it and i really had fun before with building and trying antennas

look for information on the web but always look for more information that confirms ....there's many kind of antennas(i'm from Portugal i really dont know the terms used in other languages) but in your case ur fine with a simple but cared design...and the siding for sure is not good for reception but as soon as u put the dipole above it ,pfiiiuuuu,it will add u db's of gain.....antennas always above ground plane ,and the bigger that ground plane the better the gain....

with what u say u have and what happened u should really check the line(cabling)

if u have a good ground connection near by the receiver ,find a point to connect it to ground.....

late. ...if u dont manage let me know,i'll post u a simple design and some basics...i have to go thru the stuff in my head and i dont have the books here with me,but if u catch some basics on the internet u'll be fine to go,dont forget the impedance.........
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Old 13th April 2009, 03:19 AM   #4
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It almost sounds like you've got way too much signal, not too little. With nothing attached I'd expect almost no reception. Maybe the front end is overloading and the sensitivity is way down. It might be interesting to put a pad on the input and see if things improve. My reception experience inside metal buildings is awful. The only answer is an outside antenna and a good RF ground if you can arrange it.
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Old 13th April 2009, 03:33 AM   #5
picopos is offline picopos  Portugal
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and i only used copper wire on horizontal full wave antennas,this would ocuppy something like 12meters X 12 meters(this would be mounted on outdoors,in the top of a hill,transmit to south africa,sw from california to indonesia-being in Portugal-good times)...i think u should be using some not anodized hollow metal,but then u cant make twists,the helicoidal designs that i know are for ultra directional designs....i cant see the pics u posted
i really wouldn't do an antenna with copper wire twisted in turns,,,,if so with wire,i would use it straight,and if it's on the side of the house remenber that u can only receive from the area that's on the eyesight of the antenna.....
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Old 13th April 2009, 03:37 AM   #6
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yeah check that out about the receiver
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Old 13th April 2009, 08:28 AM   #7
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Thanks for your reply. I'm really not looking for antenna suggestions, rather I'm trying to troubleshoot the system I have now.

The reason I posted this is because everything from all the research I did is supposed to be correct. Its kinda frustrating to put like 30 hours of research into something, carefully make it, and then it flops.

Anybody see where I went wrong? And does anyone have any comments on the two links with the very different Balun designs? Is one better than the other, is one totally wrong, etc?
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Old 13th April 2009, 11:04 AM   #8
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Ah, metal siding, lovely.

I live in a metal sided Farady Cage too.

CKN (Canadian Military) with an ERP of 1.3 million watts, about 30 minutes drive away pretty much disappers below the noise floor when I walk away from a window with my SW rig

My FM antenna solution is a 1/4 wave groundplane, groundplanes bent 45 degrees for a reasonable match with the tuner (measured SWR 1.5:1 @ 75 ohms ... don't ask )

Cheers!
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Old 13th April 2009, 11:33 PM   #9
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Hey John,

Couple of thoughts:

How about checking the resistance of the antenna/feedline right at the FM tuner end (using a VOM) - may have a short somewhere that is shunting away any possible signal.

Feedline seems pretty long for FM - could you mount the dipole in the attic to get the length down?

If signal actually too much as one poster suggests (though I doubt it) you could build up a 10dB or so 75 ohm pad with 3 resistors and see what attenuating the signal at the tuner input does. There are a number of attenuator calculators on the web.

Charles
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Old 13th April 2009, 11:41 PM   #10
picopos is offline picopos  Portugal
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i.ve never a seen a system were u would make that kind of coil on the line(get rid of it),there,s antennas with coils(on them,in the middle or on the base) but for matching reasons and reducing the size......and did u cut the line and then reconnect it again??i know that little details might not seem important,but i.m basing my self on dx and longer freqs and that make's a big difference.....what is the material covering the antenna.....?
and by the pic i would sugest u making a ground plane cause your antenna like that doesn't have any "reflector" to give her some gain....dont rely on the siding,too far away(impedance),connect signal to antenna and shielding to a ground plane,dont use the siding for this,try just awg wire going down and try angle it like geek refers on the ground plane,i'm sure u know the design and as u've seen u can make it ??radial?? with several reflectors 90degrees (i think that would be the best cause i still se a tree absorbing the signal in some of the antenna horizon) althoug i know u just want to solve the problem with things as they are.....well sometimes all that is dropped and probably if u extend a wire until the balcony u'll have the best signal possible h h h h h

check the line ,try just disconnecting the shield on the dipole and see the results,
i'll try to give a better look at the links....
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