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Antenna in the Attic and Coax question.

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MadSpleen85

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I recently purchased a Centerfire Deluxe Discone antenna with the hopes of mounting it on the roof. Talked to my landlord about it, after I had already bought the antenna, only to find out that he will not allow me to mount a second antenna on the roof. Already have a OTA TV antenna up there. He will however allow me to put it in the attic. Right now there is nothing in the attic besides insulation. Under the roof itself there is nothing but wood, no insulation. I have heard of people taking their scanner into the attic without an antenna and see where signal comes in the strongest.. Is that a good method of deciding where to mount or is the a different or better way?

The website that I bought the Centerfire antenna from states that it comes with a "SO-239 stud". I looked that up on Radio Shack's website, to see what exactly that is, that didn't help much as you cannot see the connections, only that it appears to be threaded and not a BNC connector. So what I am asking is, what will I need to connect coax onto this stud?

I am looking at probably a 50'-60' run of coax from the farthest corner of my attic to my desk where the scanner is. I am looking at buying 100' of RG6. I figure it is better to have more then enough and shorten to the length I need rather then not have enough. Is RG6 appropriate for this length of a run and this antenna or do I need to be looking at something different?

This setup will only be going to one scanner, RS PRO-2051, so my only connections will be at the antenna and at the back of the scanner. This will be my very first scanner antenna so I have been reading up on what other people have done, what works for them and what has not just looking to get a little bit more specific information for this setup. Any help is appreciated, some of these threads just seem to go right over my head.
 

kb2vxa

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I see no reason why you can't put a discone in the attic or use RG6 coax, you'll just need F connectors and appropriate adapters to fit the existing connectors. That "SO239 stud" is simply an SO239 connector that accepts a PL259 so you'll need an F to PL259 adapter there.

FYI, in military parlance anything with an SO designation is a socket and PL is a plug.
 

ka3jjz

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I recently purchased a Centerfire Deluxe Discone antenna with the hopes of mounting it on the roof. Talked to my landlord about it, after I had already bought the antenna, only to find out that he will not allow me to mount a second antenna on the roof. Already have a OTA TV antenna up there. He will however allow me to put it in the attic. Right now there is nothing in the attic besides insulation. Under the roof itself there is nothing but wood, no insulation. I have heard of people taking their scanner into the attic without an antenna and see where signal comes in the strongest.. Is that a good method of deciding where to mount or is the a different or better way?
<snip>
Yep that will work. Using your handheld, use either no antenna at all, or use one that's so short that it's really inefficient - like a paper clip, for example. You will need to walk carefully, as walking on beams with a radio in your hand is at best clumsy. Careful! 73 Mike
 

N2JDS

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SO and PL, Socket and Plug, HUH, I learn something new here every day, never put those two together.
 

jackj

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You might want to consider buying RG-8X cable instead of RG-6. RG-6 is 75 ohm cable and is used for TV transmission line. RG-8X is 50 ohm and matches communication antennas and radios. Also, you can buy PL-259 connectors that fit directly onto RG-8X but they won't fit directly onto RG-6.
 
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N_Jay

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Although 8X may be easier to find the right connectors for, it will be higher loss than RG-6

The impedance is irrelevant on a wide-band RX only system.
 

jackj

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Impedance doesn't matter?

Although 8X may be easier to find the right connectors for, it will be higher loss than RG-6

The impedance is irrelevant on a wide-band RX only system.
That is very interesting. I was always taught that a miss-match of impedance resulted in reflected power. In order to get the best transfer of power, the antenna should match the transmission line and the transmission line should match the radio. More power transfered equals more signal to the receiver.

You really need to stop making statements on subjects you don't know anything about N_Jay.
 

mtindor

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That is very interesting. I was always taught that a miss-match of impedance resulted in reflected power. In order to get the best transfer of power, the antenna should match the transmission line and the transmission line should match the radio. More power transfered equals more signal to the receiver.

You really need to stop making statements on subjects you don't know anything about N_Jay.
Assuming it's a receive-only setup, I'll have to agree with N_Jay.

M
 

mtindor

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I know you're only planning a 50-foot run, but if you plan on monitoring any significant 800 mhz trunked systems / conventional frequencies that are "non-local", you really should consider something with less loss, especially since you already are restricted to putting it in the attic.

I can notice a diffference on receive using 50 ft of RG58 or RG8x versus LMR-400 on 800 mhz .

Snag some LMR. It's not as flexible, it has a larger diameter, and overall it's harder to route through any obstacles, but it'll last as long as you live there with it running into your atttic and will provide you with better [lower] loss characteristcs.

If you're only monitoring 800 Mhz closer to home or are not monitoring 800 mhz at all, then don't worry about lower loss feedline.

Mike
 
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N_Jay

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That is very interesting. I was always taught that a miss-match of impedance resulted in reflected power. In order to get the best transfer of power, the antenna should match the transmission line and the transmission line should match the radio. More power transfered equals more signal to the receiver.

You really need to stop making statements on subjects you don't know anything about N_Jay.

Why don't you sweep your antenna across the band you are interested in, and sweep the input to your receiver across the same band, then get out your old Smith chart tablet and calculate the power transfer at the best and worse frequencies.
Convert all your findings into effective receive sensitivity and let me know what you find.

Why don't you learn a little before you assume you know it all.

http://www.ocarc.ca/coax.htm

Or just google and read;
http://www.google.com/search?hl=en&rlz=1R2GGLL_enUS339&q=RF+loss+calculations+impeadance&aq=f&aql=&aqi=&oq=

I know I don't know it all, but I do KNOW what I know.
 
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drsl2000

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That is very interesting. I was always taught that a miss-match of impedance resulted in reflected power. In order to get the best transfer of power, the antenna should match the transmission line and the transmission line should match the radio. More power transfered equals more signal to the receiver.

You really need to stop making statements on subjects you don't know anything about N_Jay.
For receive only, impedance mismatch is a non issue! You're better off to get a lower loss cable then worry about 50 vs 75 ohm.

The choice of cable you use is highly dependant on what you want to listen to; specifically, what frequency band you want to monitor.

I use RG6.. At 50' you are only looking at about 3 db loss at 870MHz... If you had a longer run, then you might want a better cable (LMR400).

If you are low banding it, choose any cable you like :)
 

jackj

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Why don't you sweep your antenna across the band you are interested in, and sweep the input to your receiver across the same band, then get out your old Smith chart tablet and calculate the power transfer at the best and worse frequencies.
Convert all your findings into effective receive sensitivity and let me know what you find.

Why don't you learn a little before you assume you know it all.

Coax Calculator

Or just google and read;
RF loss calculations impeadance - Google Search

I know I don't know it all, but I do KNOW what I know.
So you maintain that SWR makes no difference? Correct?

I have never claimed that RG-8X had less loss than RG-6, just that RG-6 is 75 ohm and RG-8X is 50 ohm. Over 50 years of experience in RF has taught me that reflected power will either add to or subtract from the received signal level, depending on the length of the transmission line and the frequency you are receiving. If you have to buy cable anyway, why not buy cable the connector fits and who's impedance matches the load?

You and N_Jay and anyone else who claims reflected power has no effect on received signal is full of crap! You can take that to the bank.
 

K8PBX

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While most scanners/receivers specify a 50-ohm load at their antenna jack, 75-ohm type cable can be used. Why? Because the impedance of the entire scanner/cable/antenna system as a whole changes radically over the wide range of frequencies that such radios cover.
If we were just listening to one narrow range of frequencies then we would worry about matching the cable/antenna impedance to the receiver impedance to get the most efficient signal transfer. Since we listen to signals from 25MHz to nearly 1GHz it's just not practical, so pay it no mind.
 
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N_Jay

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So you maintain that SWR makes no difference? Correct?
No. Did I say that?
It doesn't make "no difference", it just makes no relevant difference.

I have never claimed that RG-8X had less loss than RG-6, just that RG-6 is 75 ohm and RG-8X is 50 ohm.
And on a RX only system over a wide frequency range and the resulting wide termination impedances, cable loss is MUCH more important than match.

Over 50 years of experience in RF has taught me that reflected power will either add to or subtract from the received signal level, depending on the length of the transmission line and the frequency you are receiving.
It can only subtract, but the issue is does it subtract a meaningful amount?
50 years of experience may teach much, but so does and education,
The best comes from a combination of education and experience.

If you have to buy cable anyway, why not buy cable the connector fits and who's impedance matches the load?
Why not buy that cable that provides the best end result for the least cost?

You and N_Jay and anyone else who claims reflected power has no effect on received signal is full of crap!
Maybe you need to reread what we actually wrote instead of your misinterpretation.

You can take that to the bank.
Every single day.
 

KT4HX

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I totally agree with the RR brain trust on this matter. In a wide coverage receive-only application, with the variances in impedance present, using a 75 ohm cable in this case will show negligible (read - not noticeable to the listener) degradation over using a 50 ohm cable. I have put this to practical application. I currently am feeding my Drake R8A with quad-shield RG-6, even though the SO-239 on the receiver (and my PAR EF-SWL) is rated at 50 ohms. I did a test with RG-213 for the same setup and found no disecernable difference in reception quality. In a receive-only situation, cable loss is much more of a concern to me than a minor impedance bump. Especially the higher in frequency you go. FYI, for those wondering, I am using the RG-6 in this case because it is easier to bring into my house without additional modification to my entry point. If you throw transmit into the equation - now you're talking a different ballgame.
 
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Audiodave1

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Right on Alan.

I use plenty of Belden 1694A RG6 cable for my attic antennas..and a couple of UHF outdoor ones. It is worth the money.

This linked article was written by a good friend on the same subject of 50 vs 75. It is primarily concerned with cost factor but his credentials are real.
Note it speaks of wireless mics but they too are wideband receivers.
http://www.audiosystemsgroup.com/Which_Coax_for_Wireless_Mics.pdf

Dave
 
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N9JIG

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I have a plethora of scanner antennas in the attic and most are fed with decent quad shield RG-6, they work much better than the same antennas had when I used RG8, RG8X or even a couple that used 9913. You can get great deals on pre-made 50 foot assemblies of RG-6 and save a ton of money and headaches.

I got my various RF adapters from L-Comm and The RF Connection (The RF Connection Home Page). Buy a couple extra and hang on to them!

If you ever plan on transmitting thru the discone then be sure to use a 50 ohm cable like RG8 or 9913 but for RX only RG-6 is better.
 
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