Coax 50ohm or 75ohm ??

Status
Not open for further replies.
Joined
Feb 17, 2007
Messages
29
For radios I know its a big difference.

However for a scanner does the coax make a huge difference?

Looking to hook up a GRE PSR-500 to approx 50-75 ft coax cable to an 800 mhz antenna outside.
Will be used to monitor a 800MHZ P25 trunking system.

Do I need 50 or 75 ohm cable? Will using either hurt the scanner? Will either provide for better reception (not a huge deal as with just the rubber duck on top I hit 2 or 3 towers no problems).

Couldnt really find a good answer in the threads i searched.

Thanks.
 
N

N_Jay

Guest
Lowest loss is most important.
I would be highly unlikely that you would ever notice any issues with using 75 ohm cable on a scanner.
In general 75 ohm cables are lower loss, as they are designed for low signal use, while most 50 Ohm cables are designed to carry more power.
 

KT4HX

Member
Joined
Dec 19, 2002
Messages
610
Location
Spotsylvania County, Va
For receive applications only, using a 75 ohm cable will not provide any noticeable (to your ear) negative affect. When you add transmitting into the equation, then yes, it can have some noticeable affects. And, I presume that when you stated it did make a difference with radios, that you meant when used in a transmit application. N_Jay is correct that cable loss figures are more important than a minor impedance bump when used with your scanner.
 

kb2vxa

Completely Banned for the Greater Good
Banned
Joined
Mar 22, 2005
Messages
6,126
Location
Point Pleasant Beach, N.J.
One more thing, wide band scanner antennas are only resonant and thus present a 50-72 ohm terminal impedance on a few band segments here and there, anywhere else it's anybody's guess. You may have noticed that discones are advertised as being usable for transmitting in the VHF-UHF ham bands, those are their resonant points.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,131
Location
So Cal - Richardson, TX - Tewksbury, MA
A Discone's "resonant point" is about an 8:1 frequency range and you can transmit within that entire range. The match is usually fine way past the upper frequency limit but the radiation pattern pulls away from the horizon and is not very useful.
prcguy

One more thing, wide band scanner antennas are only resonant and thus present a 50-72 ohm terminal impedance on a few band segments here and there, anywhere else it's anybody's guess. You may have noticed that discones are advertised as being usable for transmitting in the VHF-UHF ham bands, those are their resonant points.
 
N

N_Jay

Guest
Now you have done it, PRC.

Warren is going to have to hit you with some of his infamous gas!
:twisted::lol::twisted:
 

kb2vxa

Completely Banned for the Greater Good
Banned
Joined
Mar 22, 2005
Messages
6,126
Location
Point Pleasant Beach, N.J.
Naaah, I'll just leave you tech heads to make another mountain out of one of my mole hills and give the casual listeners another case of indigestion with information overload long after the question has been answered.

Don't tempt me N_Jay, my gas may be natural but it doesn't come from wells.
 

jackj

Member
Joined
Jul 19, 2007
Messages
1,542
Location
NW Ohio
One thing you might want to keep in mind fyrfyterskickash is that the only difference between transmitting and receiving, besides the power level, is where the signal comes from. Power transfer is needed on both and you get the most efficient power transfer when the impedances all match. That said, 75 ohm cable does have less loss than the equivalent size 50 ohm cable. I would go with RG-8 cable for a 50-75 foot run and for use at 800 Mhz. If you feel that you have to use 75 ohm, then smaller RG-6 would work fine. Try to stay away from using connector adapters, they will mess up the return loss and increase the overall losses of your antenna system. If you have to buy cable for your antenna then let your connector requirements dictate the cable you buy. If money is no object, then go with 50 ohm 3/4" hard-line.
 
N

N_Jay

Guest
One thing you might want to keep in mind fyrfyterskickash is that the only difference between transmitting and receiving, besides the power level, is where the signal comes from. Power transfer is needed on both and you get the most efficient power transfer when the impedances all match. That said, 75 ohm cable does have less loss than the equivalent size 50 ohm cable. I would go with RG-8 cable for a 50-75 foot run and for use at 800 Mhz. If you feel that you have to use 75 ohm, then smaller RG-6 would work fine. Try to stay away from using connector adapters, they will mess up the return loss and increase the overall losses of your antenna system. If you have to buy cable for your antenna then let your connector requirements dictate the cable you buy. If money is no object, then go with 50 ohm 3/4" hard-line.
The problem with this discussion, is there is a fair amount to know, and a much greater amount to understand.

Yes, a better match transfers more power, but 75 ohm cable on a 50 ohm system is only a 1.5:1 swr. Hardly enough to worry about.
Of course that ASSumes that the radio and the antenna are actually 50 ohms.
That is a good assumption for well designed transmitters and a fair assumption for relatively narrow-band antennas.

Now, we are talking about relatively inexpensive receivers, that may or may not exhibit something 'close' to 50 ohms (I would say that 25 to 100 qualifies as "close" for a receiver.
Next we are talking about a very wide bandwidth. I would expect the receiver to not be too bad, but the antenna is likely to be all over the place.

So, what is the thought behind trying to match the cable, when the antenna is darn near a wild-ass-guess?

So lets do a little exercise
850 MHz 50 feet RG-8 perfect match = 3.97 dB loss
850 MHz 50 feet RG-8 75 ohm mismatch = 4.81 dB loss (0.84 dB from mismatch)
850 MHz 50 feet RG-6 75 ohm match = 3.51 dB loss
850 MHz 50 feet RG-6 50 ohm mismatch = 4.30 dB loss (0.81 dB from mismatch)

OK, now, why do we worry about transmitters more than receivers.

Part of the signal entering the receiver will be reflected when it encounters an impedance mismatch. This is usually a relatively small amount (Let's assume 1/2 of the power) is reflected. That reflected signal heads back to teh antenna where some of it is re-radiated (with considerable loss) and some is again reflected back to the receiver, adding (in a insignificant amount) to the original signal. The voltage and power peaks (standing waves) caused by these reflected signals are insignificant to the limitations of the cable, connectors antenna, or receiver front end.

Now for a transmitter.
Part of the signal entering the antenna system will be reflected when it encounters an impedance mismatch. This is desired to be a small amount (Let's assume 1/2 of the power) is reflected. That reflected signal heads back to the transmitter where some of it is reflected back to the antenna system but the majority of it must be dissipated in the final stage circuits of the transmitter (with considerable heat generation). In addition, the voltage and power peaks (Standing waves) caused by these reflected signals can be very significant with regard to the limitations of the transmitter final circuit elements and even the cable and connectors.

So while a mismatch on a receiver only causes the loss of a small amount of signal, the same mismatch on a transmitter can cause damage to the transmitter and/or antenna system.

On top of this, many transmitters are designed to roll-back their output power when a high SWR is detected further reducing the transmitted signal.

So, while Jack seems to KNOW something about the topic, it appears he does not UNDERSTAND all the issues and how they interrelate. (Sorry dude, I gave you several opportunities to become educated) ;)
 

n5ims

Member
Joined
Jul 25, 2004
Messages
3,690
If money is no object, then go with 50 ohm 3/4" hard-line.
You're setting your sights WAY too low for a "money is no object" coax. The Andrew LDF5-50A has good loss numbers for similar coax (LDF5-50A), but it's still a whopping 1.107 dB per 100 feet (at 800 MHz). Even with a very small antenna, you'll still need a tower to hold it up more than a few dozen feet or so.

For real "money is no object" coax, try using 6 1/8" flanged coax (Shively Labs FM Antennas and Accessories). With a loss of only 0.137 dB per 100 feet (at 800 MHz), you'll never notice the signal drop when adding an extra 300 to 400 foot to place the antenna in that "perfect" place. With the typical scanner antenna, no tower would be required (just a sturdy base mount and good guy wires) up to 100 feet or so. Of course you'd still want to use a tower, but this stuff's sturdy enough to support your normal discone without one!
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,131
Location
So Cal - Richardson, TX - Tewksbury, MA
Yea, but what if part of your mole hill answer was wrong information?
prcguy

Naaah, I'll just leave you tech heads to make another mountain out of one of my mole hills and give the casual listeners another case of indigestion with information overload long after the question has been answered.

Don't tempt me N_Jay, my gas may be natural but it doesn't come from wells.
 

jackj

Member
Joined
Jul 19, 2007
Messages
1,542
Location
NW Ohio
N_Jay, you totally misrepresent what I said while you say about the same thing. One mistake I made was that I didn't specify that I was assuming the transmission line losses would be the same when I said "Power transfer is needed on both and you get the most efficient power transfer when the impedances all match." I figured that folks would be smart enough to know that other variables were NOT included in that statement. I guess I was wrong, you couldn't understand that. Then again, maybe you boost your ego by trying to belittle others.

fyrfyterskickash don't pay any attention to N_Jay, all he wants to do is argue. Reread what I posted earlier about selecting transmission line. Good luck and I wish you well.
 
N

N_Jay

Guest
fyrfyterskickash don't pay any attention to N_Jay, all he wants to do is argue. Reread what I posted earlier about selecting transmission line. Good luck and I wish you well.
Well, that is good advice. :roll:

Please do read the whole thread. (And this one) and then make up your own mind.
 
Joined
Dec 26, 2004
Messages
1,217
Location
Tulsa
So much mis-information, futile to try to educate. In the relm of RF transmission theory where does 50 Ohms come into play? Let's pile some more wood on the fire and really let it blaze!
 

W6KRU

Member
Premium Subscriber
Joined
Nov 20, 2008
Messages
3,390
Location
Oceanside, CA
So much mis-information, futile to try to educate. In the relm of RF transmission theory where does 50 Ohms come into play? Let's pile some more wood on the fire and really let it blaze!
Yeah, sometimes it's like trying to convince some people that they don't need 18' of coax in their mobile CB installation.
 
Last edited:
Status
Not open for further replies.
Top