Transmission Line question.

[musiconradio.com]

Question for the experts.

I have 320' of 1 5/8 transmission cable for our FM. We are a class A and will be using the full 100M. With the antenna factored in and our transmitter shack right next to the tower, will this be okay, if not. What are the options to make it work. Thanks

[RGORJANCE]

This is a very quick and rough calculation on my part.

It looks like you will need about 4.8kw out of the xmttr into a three bay circular antenna to make 6kw H and V. The three bay has a nominal gain of 1.480 X power arriving at the antenna of about 4kw (assuming line eff of 85%).

The equation changes if you use a 3 kw xmttr (I did not calculate the Tpo for the 3kw rig) and a 6 bay antenna. The other part of the equation is the capability of the tower to hold the additional weight and wind load of the 6 bay.antenna. Obviously the long term cost factor is the power consumption of the two xmttrs. The 3kw rig will consume less power, and be more cost effective in the long run.

Get us frequency, etc and I can calculate it out much more accurately for you.

Fossil

[Deep Thought]

Bob...this is it: http://transition.fcc.gov/fcc-bin/fmq?list=0&facid=189541

Your CP is for 3.4 KW ERP. Previous posts also imply you have a 5 KW BE transmitter. With 1-5/8" air dielectric line you'll need:

3-Bay:

Code: Select all

 1.  3.4        Eff. Radiated Pwr in kW  14.  325 ft.      Length of Line
 2.  96.3       Frequency in MHz         15.  1-5/8" Air   Type of Line
 3.  3          Number of Bays           16.  0.2003 dB    Attenuation Per 100'
 4.  1.5588     Antenna Power Gain       17.  NONE         Other Losses in dB
 5.  1.25       Antenna Field Gain       18.  86.08 %      Line Efficiency
 6.   2.181 kW  Antenna Input Power      19.  0.353 kW     Power Loss in Line
 7.  172 mV/m   Ant. FI @ 1 mi./1kw      20.   2.534 kW    Transmitter Pwr Out
 8.  End        Center or End Feed       21.  105          Hangers Required
 9.  20.44 ft.  Length of Antenna        22.  2            Hoist Grips
10.  341 ft.    Height of Top Bay        23.  19.6 Deg.    Angle to 1st Null
11.  331 ft.    Cntr of Rad (AGL)        24.  0.180 Miles  Dist. to 1st Null
12.  NO DATA    Cntr of Rad (HAAT)       25.  42.0 Deg.    Angle to 2nd Null
13.  10 ft.     Distance to Tower        26.  0.070 Miles  Dist. to 2nd Null


2-Bay:

Code: Select all

 1.  3.4        Eff. Radiated Pwr in kW  14.  329 ft.      Length of Line
 2.  96.3       Frequency in MHz         15.  1-5/8" Air   Type of Line
 3.  2          Number of Bays           16.  0.2003 dB    Attenuation Per 100'
 4.  .9971      Antenna Power Gain       17.  NONE         Other Losses in dB
 5.  1          Antenna Field Gain       18.  85.92 %      Line Efficiency
 6.   3.410 kW  Antenna Input Power      19.  0.559 kW     Power Loss in Line
 7.  137.6 mV/m Ant. FI @ 1 mi./1kw      20.   3.969 kW    Transmitter Pwr Out
 8.  End        Center or End Feed       21.  106          Hangers Required
 9.  10.22 ft.  Length of Antenna        22.  2            Hoist Grips
10.  335 ft.    Height of Top Bay        23.  30.0 Deg.    Angle to 1st Null
11.  330 ft.    Cntr of Rad (AGL)        24.  0.110 Miles  Dist. to 1st Null
12.  NO DATA    Cntr of Rad (HAAT)       25.  90.0 Deg.    Angle to 2nd Null
13.  10 ft.     Distance to Tower        26.  0.000 Miles  Dist. to 2nd Null

Single Bay:

Code: Select all

 1.  3.4        Eff. Radiated Pwr in kW  14.  322 ft.      Length of Line
 2.  96.3       Frequency in MHz         15.  1-5/8" Air   Type of Line
 3.  1          Number of Bays           16.  0.2003 dB    Attenuation Per 100'
 4.  .4611      Antenna Power Gain       17.  NONE         Other Losses in dB
 5.  .68        Antenna Field Gain       18.  86.20 %      Line Efficiency
 6.   7.374 kW  Antenna Input Power      19.  1.181 kW     Power Loss in Line
 7.  93.6 mV/m  Ant. FI @ 1 mi./1kw      20.   8.555 kW    Transmitter Pwr Out
 8.  End        Center or End Feed       21.  104          Hangers Required
 9.   0.00 ft.  Length of Antenna        22.  2            Hoist Grips
10.  318 ft.    Height of Top Bay        23.  90.0 Deg.    Angle to 1st Null
11.  318 ft.    Cntr of Rad (AGL)        24.  0.000 Miles  Dist. to 1st Null
12.  NO DATA    Cntr of Rad (HAAT)
13.  10 ft.     Distance to Tower

(courtesy of the ancient but excellent CEI tech calculator)

Looks like you could comfortably do this with a two bay full wavelength spaced antenna. 320 feet of line is barely going to make it though.

[musiconradio.com]

Thanks for the info.

[RGORJANCE]

Thanks Deep. I was really working off the top of my head.

With only 320 ft of line, the building is going to have to be really close to the tower and the xmttr is going to have to be real near the wall to get it done with a 2 bay and that line length. I thought he was a full 6kw A.

He should have enough line to get into the building. If he runs short, running a couple pieces of rigid and clamp flanges would be a quick and somewhat economical solution.

Fossil

[RFWarrior]

320 feet of line is barely going to make it though.

Deep's numbers very closely mirror mine - I get 3.963kW into a 2 bay, full wave spaced, end fed antenna and 3.973kW into a center fed. Distance wise, depending on if the antenna is end fed or center fed, you'll have 303 feet or 308 feet from the feed point to ground. If you bring the coax over to the building at the 7 foot height, you've got around 15-20 feet of horizontal length to get from the tower to the transmitter output. Or, as Bob said, get the coax just into the building, install a coax ground kit on it (your transmitter will thank you for that, regardless of brand name ) and use hardline for the "in the building" portion - costs a bit more, but makes for a much cleaner installation. Don't forget that if you're pressurizing the line, or using nitrogen, you'll need a gas block connector on the bottom end, along with the appropriate fittings. I'm not an expert, but doing neither probably shouldn't be an option - others with more experience can speak to that

Best,

Jeff