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Radio Antenna

 
 
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ad90
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PostPosted: Mon May 14, 2007 10:24 pm    Post subject: Radio Antenna Reply with quote

Question,

Can I use a Springer CB antenna for radio (FM/MW) reception? I know I will have to splice the coax, but is there any other reason it won't work? Failing that do you get a similar styled antenna for radio reception?

Thanks

AD90
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.
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PostPosted: Tue May 15, 2007 7:51 am    Post subject: Reply with quote

Tx/Rx aerials worth slightly differently from Broadcast Rx aerials but thats not the real problem - you can't just "splice" the co-ax as you suggest.

Every time you transmit on the CB you will actually transmit into your radio and will blow the crap out of it Also you will affect the antenna balance and will eventually also wreck the CB.

The proper way to combine these is to use a diplexer. A good one would cost you about £100!

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needalanny
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PostPosted: Tue May 15, 2007 9:08 am    Post subject: Reply with quote

Out of curiosity Ian, is it possible to use a CB ariel (trucker 80 for example) as a soley dedicated FM/AM ariel and then a seperate one for the CB?

Will it pick up the correct frequency, or cause damage to the radio?

I know what curiosity did to the cat, but i can't help it.....

Matt
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ad90
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PostPosted: Tue May 15, 2007 11:24 am    Post subject: Reply with quote

Thanks Ian,

What I wanted to do is what Needalanny described. i.e. 2 antenna 1 CB and 1 CB type used for Broadcast Rx only.

From what I gather, this should be OK?

Cheers

AD90
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PostPosted: Tue May 15, 2007 1:06 pm    Post subject: Reply with quote

Without going into a whole technical explanation about wavelengths, resonance, capacitance, etc the simple answer is no you can't use a CB aerial to receive broadcasts although it will work a bit on FM where the signal is strong (even a coat hanger will work in good signal areas for FM!) but definately won't work on MW.
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???? the V8 90 CSW
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ad90
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PostPosted: Tue May 15, 2007 2:10 pm    Post subject: Reply with quote

Thanks Ian,

Loud and clear - unlike my radio Smile

To my original quest then; does anyone know where I can get a springer lookalike for suitable for Broadcast Rx?

AD90
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mrcheese
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Joined: 14 Oct 2003
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PostPosted: Wed May 16, 2007 2:07 pm    Post subject: Reply with quote

I am not sure that I agree with Ian's response. He is correct when talking about an antenna that is used for TX and RX. If you have a a good TX antenna it will make a good RX antenna on the same band and vice versa.

RX only is a different matter when talking about an antenna that is used for RX on a broad range of frequencies as seen in a car radio.

A typical car radio will receive:

LW - Around 500KHz
MW - Around 1MHz
FM - Around 100MHz
(CB is 27MHz)

A car radio antenna is just a length of wire. It is not in itself matched to the frequency in the same way a CB antenna is set up. Any matching is carried out inside the radio. A common technique is an active antenna. This is what is used on many car radios. This matches the impedance and provides amplification. OK, I accept that this is not efficient, but broadcast stations have outputs in the KW to MW range (4W for CB). This is why the "coathanger" works well as a replacement.

Regards,

Paul.
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mrcheese
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PostPosted: Wed May 16, 2007 3:15 pm    Post subject: Reply with quote

Correction - LW is around 200KHz.

A bit of information on the active antenna can be found here:

http://www.southgatearc.org/qrp/activeantenna.htm

I also remember some older car radios. On the back was a small capacitor that you adjusted for best receive on FM.

Paul.
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PostPosted: Wed May 16, 2007 3:18 pm    Post subject: Reply with quote

There is more to it than just the length of the antenna and frequencies used. Car stereos use a low capacitance 75 ohm co-ax to allow them to receive LW & MW transmissions whereas a CB or any other PBR transceiver uses a 50 ohm matched antenna and co-ax. If you use a PBR band 50 ohm antenna and co-ax there is next to no signal left by the time it reaches the receiver.
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mrcheese
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PostPosted: Wed May 16, 2007 4:31 pm    Post subject: Reply with quote

Sorry, but I am going to disagree again.

Coax only presents minimum loss when correctly matched. So in a PBR application the TX is designed to work into a 50 ohm system. For max efficiency the 50 ohm coax needs to be connected to an antenna that presents 50 ohms impedance. A quarter wave vertical is close to this when fed against ground ( the car). A typical car antenna will only get near this on FM. On LW and MW the impedance will be from hundreds to thousands of ohms. For this reason the impedance of the connecting cable is not relevant. especially as given the short length used. You could measure it, but not hear it.

The capacitance of the cable is generally a product of the design which gives the characteristic impedance. 50 ohm is about 30pF/foot and 75 ohm about 20pF/foot. Assuming 6' of cable, 60pF difference will have no effect at LW/MW


Our obsession with certain cable impedances goes back to experiments made by Bell Labs. They tried to determine which impedance was best for a particular application.

50 ohm - 30 to 40 ohm is optimal for high power. However, this is not easy to make so 50 ohm was the compromise.

75 ohm - 77 ohm was found to be the lowest loss type. Good for long distance use. Used as a standard for TV, video and telecoms. 72 ohms is the impedance of a center fed dipole.

93 ohm - You may have seen this on old IBM terminals. Low capacitance which is good for digital, but seldom seen now.

50 ohms is not magic. We just use it as a standard and design accordingly.

Regards,

Paul.
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PostPosted: Wed May 16, 2007 5:15 pm    Post subject: Reply with quote

Whatever

Ian "Been there done that and knows a CB aerial won't work for MW" Culbert

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Bert the Defender 110 XS - because it's Cool (work it out yourself!)
Lolita the Lightweight
???? the V8 90 CSW
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mrcheese
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PostPosted: Wed May 16, 2007 6:02 pm    Post subject: Reply with quote

Sorry if I am boring you. No need to take it personally!

Paul "been there, there done that, can make 6" of wet string work" Sayer
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Winnet
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PostPosted: Wed May 16, 2007 7:50 pm    Post subject: Reply with quote

I have just found a dummy and a rattle bouncing down the road, could I use them to receive MW on my crystal radio set?

Mr. Green Mr. Green Mr. Green Mr. Green Mr. Green Mr. Green Mr. Green Mr. Green Mr. Green Mr. Green Mr. Green Mr. Green

G.

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mothyb
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PostPosted: Wed May 16, 2007 11:34 pm    Post subject: Reply with quote

well i have to say that i know nowt about the technicality's of this stuff but having lived in the Scottish borders for a few years where the reception on the radio is pants at the best of times, and not needing the CB anymore, i naively wired my mag-mounted CB ariel into the radio which was a huge improvement in quality to the standard car ariel. not sure about MW and LW (never listen to em) Very Happy so in my experience, yes a CB ariel will work
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Winnet
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PostPosted: Thu May 17, 2007 7:51 am    Post subject: Reply with quote

I think we have all tried the trusty coat hanger trick on a car in the past so a CB ariel may well work for FM (give it a try mate). It just depends on how much he listens to AM/MW progs like Radio Five.

G.

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ad90
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PostPosted: Thu May 17, 2007 4:02 pm    Post subject: Reply with quote

Well, since I almost never listen to MW/LW I am going to give it a try.

AD90
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Winnet
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PostPosted: Thu May 17, 2007 4:13 pm    Post subject: Reply with quote

Does 'Fit-like' radio not broadcast on MW then???

G.

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ad90
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PostPosted: Thu May 17, 2007 9:38 pm    Post subject: Reply with quote

Waves Radio = Radio Huddicks
NECR = Neeps on the wireless
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Winnet
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PostPosted: Thu May 17, 2007 10:10 pm    Post subject: Reply with quote

And lets not mention Norfsound 2.

G.

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PostPosted: Fri May 18, 2007 8:46 am    Post subject: Reply with quote

Quote:
No need to take it personally!


I didn't - I just know what works and what doesn't so we'll just have to agree to differ on this one. Very Happy

BTW I too can make a bit of wet string receive Laughing

Ian "HND in Electronics and Telecommunications" Culbert

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Bert the Defender 110 XS - because it's Cool (work it out yourself!)
Lolita the Lightweight
???? the V8 90 CSW
Suzuki DL1000 V-Strom - yes that's right, I have a Zook!
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ad90
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PostPosted: Fri May 18, 2007 4:30 pm    Post subject: Reply with quote

All I want is something that will take a few twangs off trees and bushes and that if it gets really bad, removable. And give reasonable performance on FM (anything must be better than the current 'rubber duck').

AD "Born Chancer" 90
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PostPosted: Sat May 19, 2007 8:32 am    Post subject: Reply with quote

I have quite robust fibreglass Rx antennas which are actually designed for use on lorries. There is a certain amount of flex in them but they are easily removed if required. I use one on the 90 and it hasn't broken yet. Drop me a PM if yoiu want a price or more info
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Bert the Defender 110 XS - because it's Cool (work it out yourself!)
Lolita the Lightweight
???? the V8 90 CSW
Suzuki DL1000 V-Strom - yes that's right, I have a Zook!
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ad90
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PostPosted: Sun Jun 03, 2007 9:03 pm    Post subject: Reply with quote

Ian,

Did you get my PM?

AD90
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PostPosted: Mon Jun 04, 2007 8:14 am    Post subject: Reply with quote

Hi Andrew - Just checked not sure how I missed it. Will get you a reply today
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smokeyjoe
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PostPosted: Thu Jun 07, 2007 8:23 am    Post subject: Reply with quote

ad90 wrote:
All I want is something that will take a few twangs off trees and bushes and that if it gets really bad, removable. And give reasonable performance on FM (anything must be better than the current 'rubber duck').

AD "Born Chancer" 90


After years of fiddling with aerials I must agree with Ian.

A good is the flexi roof aerials fitted to most modern stuff. Get down the breakers yard.

I product test I saw years back compared all the expensive aerials that were available at the time, ranging from £10-£60. As a 'control' they also included a coat hanger bent into a heart shape.

They tested with meters, scopes & general driving. The Bosch came 1st, the coat hanger 2nd.

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