Amateur radio operators, got a question... (N/A)

wbarnhill

wbarnhill

Final Approach
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Feb 26, 2005
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Greenwood, SC
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iEXTERMINATE
I'm going to take the General test tonight I think. I've taken five practice tests and gotten 90%+ each time. Gonna keep taking them until about 5:30, then go up to the test center at 7:00.

There is one question I'm having trouble understanding...

What percentage of power loss would result from a transmission line loss of 1 dB?
A. 10.9 %
B. 12.2 %
C. 20.5 %
D. 25.9 %

What makes the answer to this question C) 20.5%? I'm missing a calculation somewhere.

Thanks!

William (KI4CLM)
 
wbarnhill said:
I'm going to take the General test tonight I think. I've taken five practice tests and gotten 90%+ each time. Gonna keep taking them until about 5:30, then go up to the test center at 7:00.

There is one question I'm having trouble understanding...

What percentage of power loss would result from a transmission line loss of 1 dB?
A. 10.9 %
B. 12.2 %
C. 20.5 %
D. 25.9 %

What makes the answer to this question C) 20.5%? I'm missing a calculation somewhere.

Thanks!

William (KI4CLM)
Click to expand...
it is a non-linear loss. So while 3dB is 50% power loss you cannot just use a linear solution to figure out the loss of 1dB.

The equation is

LP = 10 * Log10[P/P0]

Which in this case is

-1dB = 10 * Log10[P/P0]

P/P0 is the power ratio

10-.1 = P/P0 =.7943

Change that to a percentage, 79.43% and we can see that you have lost 100% - 79.43% = 20.56%

73 es GL de K9PO
 
Just FYI

KI4CLM /AG

:D

Now to work on my Extra, and maybe by the time I get that, I'll have some cash to buy a base rig so I can play on HF...
 
Ok now that you know how to do this I can share with you some approximations that are easy to remember

Loss 3dB is 50% or 1/2
Loss of 2dB is 33% or 1/3
Loss of 1 dB is 20% or 1/5

These are just rough numbers that you can easily use to get a number close to the real value.
 
If I run multiple receivers (air-band) off one antenna, do I need more than just a splitter? Will there be a net loss in signal to each radio if only a splitter is used?
 
TangoWhiskey said:
How do you fix that? A pre-amp? If so, before or after the split?
Click to expand...

Depends. Generally a pre-amp won't help much unless it's "tuned" because it will try to amplify everything the antenna put's out and will become overloaded if there's a strong transmission present. What's the reason for the multiple receivers? Unless you're trying to receive on all of them at the same time you'd be better off with a switch or relay. And if they cover different bands you can use a frequency selective splitter which has much less loss since theoretically you can send all the signal in one band to one receiver.
 
wbarnhill said:
Just FYI

KI4CLM /AG

:D

Now to work on my Extra, and maybe by the time I get that, I'll have some cash to buy a base rig so I can play on HF...
Click to expand...

Good job. You'll find the Extra written to be a bit more of a challenge. Back in the "old days" when there was still an Advanced written it was the most difficult written exam. The Extra written could be passed simply by playing in the hobby for a while and paying attention. 20 wpm was the real gate. Now the question pools for Advanced and Extra have been merged. I saw questions in the Advance written back in 1992 that covered stuff I hadn't seen since college (BSEE). So, while they've done away with the code, they still aren't just giving away Extra tickets. But, you'll get it.

When you get on HF, let us know. 80 or 40 meters ought to do the trick to the PNW.

73

Ghery, N6TPT
ARRL TA
 
TangoWhiskey said:
How do you fix that? A pre-amp? If so, before or after the split?
Click to expand...
See Lance's post.

But you may not need to fix anything. Most receiver are very sensitive and have more than enough head room such that the loss is not that big a deal for most applications. I have a splitter on Number 2 comm in my plane. This allows me to plug my handheld into the antenna. I have yet to be anywhere where what I could hear on comm1 could not be heard on comm 2. There is simply enough power being received that the 3dB loss is still within the receviers range to 'hear'.


The easiest way to 'fix' it is not through a pre-amp. pre-amp can make more trouble than they are worth. I find it much easier to use a gain antenna. But of course this is again going to be application dependent.
 
If you guys want to get on a chat sometime count me in...

denny-o
K8DO
 
smigaldi said:
See Lance's post.

But you may not need to fix anything. Most receiver are very sensitive and have more than enough head room such that the loss is not that big a deal for most applications. I have a splitter on Number 2 comm in my plane. This allows me to plug my handheld into the antenna. I have yet to be anywhere where what I could hear on comm1 could not be heard on comm 2. There is simply enough power being received that the 3dB loss is still within the receviers range to 'hear'.


The easiest way to 'fix' it is not through a pre-amp. pre-amp can make more trouble than they are worth. I find it much easier to use a gain antenna. But of course this is again going to be application dependent.
Click to expand...

I found a great sticky thread on this that educated me further:

http://www.radioreference.com/forum...5054-using-one-antenna-multiple-scanners.html

Looks like a good active multicoupler (like the 4- or 8-way VHF/UHF from Stridsberg) would be the best solution.

You asked the "reason"; I'm setting myself up as a LiveATC.net feed for some local airports. I'll have (eventually) multiple receivers receiving/serving up discrete frequencies. I only intend to put up one VHF omni-directional airband antenna, though I may do a directional to grab the ground / clearance delivery at the local field (KFTW) if I don't get good reception off the omni.
 
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TangoWhiskey said:
I found a great sticky thread on this that educated me further:

http://www.radioreference.com/forum...5054-using-one-antenna-multiple-scanners.html

Looks like a good active multicoupler (like the 4- or 8-way VHF/UHF from Stridsberg) would be the best solution.

You asked the "reason"; I'm setting myself up as a LiveATC.net feed for some local airports. I'll have (eventually) multiple receivers receiving/serving up discrete frequencies. I only intend to put up one VHF omni-directional airband antenna, though I may do a directional to grab the ground / clearance delivery at the local field (KFTW) if I don't get good reception off the omni.
Click to expand...

The biggest loss item is cable loss. Use a good coax and keep the lines short as possible.
 
smigaldi said:
The biggest loss item is cable loss. Use a good coax and keep the lines short as possible.
Click to expand...

What's a "good coax" for the 118-135MHz band? I find radios, pre-amps and other other items using both BNC and N-type connectors; any preference there for minimal signal loss? I thought N-type was designed specifically for microwave use; any benefits when used in the lower bands?
 
TangoWhiskey said:
What's a "good coax" for the 118-135MHz band? I find radios, pre-amps and other other items using both BNC and N-type connectors; any preference there for minimal signal loss? I thought N-type was designed specifically for microwave use; any benefits when used in the lower bands?
Click to expand...
I use N-connectors sometimes at VHF but always at UHF and higher. How long are you antenna runs going to be? If we are talking about 10-25 feet RG58A is probably good enough, the loss is about 4.5DB/100 feet at 100MHz and 6dB per 100ft at 200 MHz. With a 3dBi antenna and your multicoupler and you are probaly in the ballpark of a 4-6dB system loss. Not bad for VHF. Given the high power nature of ATC you will probably be ok. How far are you from their transmitters?

But anylonger than 25ft and I go to either 9913 or LMR400 from Times, which, IMHO, is a little easier to work with than the 9913.

BTW in my VHF/UHF mobile set up I use RG58. It is jsut easier to work with than the thicker stuff.

On my weak signal VHF set up I use a lot of LMR400, a 14 element beam that gives me a little over 10dB of gain. But that is way too much overkill for what you are trying to do.

My UHF set up is 36 elements and my 2.4GHz is a 60cm dish with a downconverter at the feedpoint. That gives me so much power I run it through RG59 (75ohm) and a 6dB attenuator to stop from over loading the front end of the recevier. :yikes:
 
smigaldi said:
I use N-connectors sometimes at VHF but always at UHF and higher. How long are you antenna runs going to be? If we are talking about 10-25 feet RG58A is probably good enough, the loss is about 4.5DB/100 feet at 100MHz and 6dB per 100ft at 200 MHz. With a 3dBi antenna and your multicoupler and you are probaly in the ballpark of a 4-6dB system loss. Not bad for VHF. Given the high power nature of ATC you will probably be ok. How far are you from their transmitters?
Click to expand...

The office I'll be hosting the radios/servers from is upstairs, and the antenna will be in the attic initially; may put an outside roof mast on the chimney later if the interior mount ends up not being good enough reception.

The initial antenna I have ordered has a 2.6db gain, and an integrated 18 foot RG8X cable with a BNC-male connector. The 18-feet should be sufficient for me to make a direct drop from the rafters into my office space and the scanner. As I add scanners later, I can add a multicoupler at the end of that 18-foot drop, with short (less than 8 feet) coax connections from the MC to the individual radios.

My house is 6.5437 statute miles from the control tower at KFTW, as the bird flies.

smigaldi said:
But anylonger than 25ft and I go to either 9913 or LMR400 from Times, which, IMHO, is a little easier to work with than the 9913.

BTW in my VHF/UHF mobile set up I use RG58. It is just easier to work with than the thicker stuff.

On my weak signal VHF set up I use a lot of LMR400, a 14 element beam that gives me a little over 10dB of gain. But that is way too much overkill for what you are trying to do.

My UHF set up is 36 elements and my 2.4GHz is a 60cm dish with a downconverter at the feedpoint. That gives me so much power I run it through RG59 (75ohm) and a 6dB attenuator to stop from over loading the front end of the recevier. :yikes:
Click to expand...

Thanks for all the help!

Amazing the difference in dB loss/100ft at 100Mhz in different cable types:
http://www.econ2way.com/catalog/public/recommended_cable.htm
 
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TangoWhiskey said:
The office I'll be hosting the radios/servers from is upstairs, and the antenna will be in the attic initially; may put an outside roof mast on the chimney later if the interior mount ends up not being good enough reception.

The initial antenna I have ordered has a 2.6db gain, and an integrated 18 foot RG8X cable with a BNC-male connector. The 18-feet should be sufficient for me to make a direct drop from the rafters into my office space and the scanner. As I add scanners later, I can add a multicoupler at the end of that 18-foot drop, with short (less than 8 feet) coax connections from the MC to the individual radios.

My house is 6.5437 statute miles from the control tower at KFTW, as the bird flies.



Thanks for all the help!

Amazing the difference in dB loss/100ft at 100Mhz in different cable types:
http://www.econ2way.com/catalog/public/recommended_cable.htm
Click to expand...
Ya you should be fine with that. I would not even worry about the pre-amp idea at all.

BTW RG8X loss at 150MHZ is 4.7dB/100ft just in case you were wondering. Lots of hams use that for VHF work.
 
Welp, I'm addicted to HF now.

Went to a friend's house yesterday to try out HF for the first time on his rig (IC-7000, running 500W on a dipole). After scanning for 20 minutes or so, we found a guy in the Ukraine, and my friend (KK6AA) got a contact, then asked if I wanted to pick up the guy as well. I passed hoping that I'd get a contact on my own. Finally after trying a few CONUS contacts that were too busy chatting with each other to respond, we found a guy in Italy (IZ0HUA), and after about three attempts, I heard the wonderful "Kilo India Four, again?", and got through. So my logbook officially has one entry now:

Freq - 14.320 MHz
Mode - SSB
Date - 5 June 2009
Time - 2236Z
Callsign - IZ0HUA
Name - Rob
Location - Rome, Italy
Signal Report - 5/5

On another good note, I no longer have to run the exhaustive "temporary AG" after my call, as the FCC ULS updated last night to reflect the change.

This is way too much fun. :D
 
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