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Reducing output signal   Message List  
Reply | Forward Message #5582 of 6131 |
Re: [LPFM] Re:Reducing output signal

Unfortunately it's not as simple as working it out to a mW per meter. That's why things are done in dB.
 
For example, driving a coax cable with a known loss of say 3dB for a given frequency with a 500mW TX, means 250mW is possible through the cable.  Since 3dB loss (-3dB) would be the RF output power divided by two. If it was gain like a 3dB antenna then it's the opposite, times 2.
 
Driving it with a 1 Watt transmitter /2 however means 500mW will be possible depending on antenna characteristics etc...
 
6dB of loss on the other hand would mean the available signal through the same cable type, freq used etc... is four times more loss.
 
Here's some calculators for RF stuff in html format. Some might find them useful to have up their sleeve. One's a cable loss one the other working out power budgets. They're just one's I've saved off the net for my own personal use.
 
RG-58A's loss at 88.4MHz is 13.617dB per 100 meters. Ewwww (yes good for wacking back power).
 
 
I like RG-213 for LPFM and Altelicon CA-400 for microwave use.
 
Cheers,
Gavin.
 
 
 
 
----- Original Message -----
Sent: Friday, February 15, 2008 5:56 AM
Subject: Re: [LPFM] Re:Reducing output signal

Hey Johnno,
 
Studying your figures, it seems that RG58A has a loss of 13mW per metre? So would I be right in saying to get a 1 watt TX down to approx. 300mW or less I would need around 60 metres of RG58A?
 
Cheers, Mike
 
 
On 15/02/2008 12:01:31 a.m., Johnno (valentine@xtra.co.nz) wrote:
> Hello Grant,
>
> Will be a little loss in TX power... now to get my calculator out :)))
>
> If your TX is out putting 500mw @ 88.4 MHZ, SWR of 1:1 at 7 meters of
> coax
> will be
>
> RG58 = Output 409 mw
> RG58A = Output 401 mw
> RG58C = Output 384 mw
> RG213 = Output 450 mw
>
> If you have already increased TX to 1 watt for the 87.5 - 88.4 as RSM is
> talking about then the following applies..
>
> RG58 = Output 818 mw
> RG58A = Output 803 mw
> RG58C = Output 768 mw
> RG213 = Output 901 mw
>
> I hope that helps...
>
> Johnno
>
> ----- Original Message -----
> From: "88.4 FM The Cheese" <studio@thecheese.co.nz [link:
> mailto:studio@thecheese.co.nz]>
> To: <LPFM_Radio@yahoogroups.com.au [link: mailto:LPFM_Radio@yahoogroups.
> com.au]>
> Sent: Thursday, February 14, 2008 10:46 PM
> Subject: [LPFM] Re:Reducing output signal
>
> > This is of interest to me, as I need to move my transmitter for a few
> > weeks while decorating the studio.
> > I have a 7 metre coax extension lead. Will this cause much loss to my
> > transmitter power?
> >
> > Cheers,
> > Grant
> >
> >
> >



Fri Feb 15, 2008 2:10 am

kiwi_rock_24
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Power Budget
   
Transmit Power Antenna Gain Power
 mW
OR

 dBm
 dB(i)  dB
   
Cable Properties
(per 100 ft or m)
Cable Length  
 dB (100 ft)
OR
 dB (100 m)
 feet
OR

 meters
 dB
   
Frequency in MHz No. of Connectors  
 MHz.  No.  dB
   
   dB

 

Free Space Loss

Loss of power over distance (assuming no FRESNEL Effect and nothing in the way). This a very idealised calculation and in practice everything interferes with the signal but it will give you a reasonable approximation of the actual loss over distance.

Enter the system Frequency in MHz and the distance in either Kilometers (Km) or Miles below and then click the 'Calculate' button. 1 GHz = 1000 MHz e.g. 2400 = 2.4 GHz.

Calculations and equations used.
  Result
 MHz  Km
OR
 Miles
 dB

 

Fresnel Zones

Defines how much clearance you need (yes you need more than simple Line-of-Sight) and for longer links > 3 Km (2 miles) whether you may have a ground clearance problem from our friendly planet. For more info on M. Fresnel and his theories go here.

Enter the Total link distance (in Miles or Kilometers), if you do not enter an Obstacle distance (in Miles or Kilometers) the calculator will use the mid-point for all calculations (Note: assumes antennas at same height). Finally enter the system Frequency in MHz and then click the 'Calculate' button. 1 GHz = 1000 MHz e.g. 2400 = 2.4 GHz.

The calculator will generate the radius of the 1st Fresnel zone only (at the obstable point or the mid-point), the 60% (no obstacle) radius and the height of the earth curvature at the mid-point of the Total link distance.

Calculations and equations used.
 Km
OR

 Miles
 Km
OR
 Miles
 m
at
 Km
 ft
at
 Miles
 
 MHz    m  ft
 
     m  ft

 

System Performance

This calculator will give you one of three answers:

  1. If you leave distance blank it will apply the defined SAD factor (or default to 30%) to the Operating Margin and supply the max. distance (in Km and Miles) at which the Margin operates.
  2. If you enter the distance it will calculate the Operating Margin and the SAD factor.
  3. If you enter distance but leave RX or TX antenna gain (or both) blank it will apply the chosen SAD factor (or default it to 30% if none supplied) and generate the required antenna power. If both are left blank it will calculate a symmetrical antenna gain.
To RESET any parameter above just set to BLANK before clicking 'Calculate'

Notes: RX Sensitivity is ALWAYS expressed as a negative dBm (- dBm) and is the lowest power of signal your radio can handle. Its buried somewhere in your radio spec and will be typically in the range of -80 to -110 dBm. Don't guess or 'fudge' this number.

Calculations and equations used.
  Result
 MHz  Km
OR  Miles
   dB
 
 dBm
OR
 mW
 dBm  dB  dB
 
 dBm  dB  dB  dB
RX Power SAD Factor Theoretical Margin
 dBm  %  dB

 

milliWatts to dBm (and vice versa)

Power in milliWatts to dBm (and vice versa) . Enter the 'Transmit Power' (A or G above) in milliWatts OR the 'Power Ratio' in dBm and click the appropriate 'Calculate' button. 1 Watt = 1000 milliWatts.

Calculations and equations used.
  Result
mW/W  dBm/W/i
  Result
 dBm/W/i  mW/W

      


Types of Coax Cable and Line Loss Calculator

 

  • Belden
    • Belden 8215    (RG-6A)
    • Belden 8237    (RG-8)
    • Belden 9913    (RG-8)
    • Belden 9258    (RG-8X)
    • Belden 8213    (RG-11)
    • Belden 8261    (RG-11A)
    • Belden 8240    (RG-58)
    • Belden 9201    (RG-58)
    • Belden 8219    (RG-58A)
    • Belden 8259    (RG-58C)
    • Belden 8212    (RG-59)
    • Belden 8263    (RG-59B)
    • Belden 9269    (RG-62A)
    • Belden 83241   (RG-141A)
    • Belden 8216    (RG-174)
    • Belden 8267    (RG-213)
    • Belden 9913F7   
    • Belden 7810A   
    • Belden 7808A   

  • Davis RF
    • Davis RF Bury-Flex

  • Times Microwave Systems
    • TMS LMR-100A
    • TMS LMR-200
    • TMS LMR-240
    • TMS LMR-400
    • TMS LMR-600
    • TMS LMR-900

  • Wireman (coax)
    • Wireman CQ102    (RG-8)
    • Wireman CQ106    (RG-8)
    • Wireman CQ125    (RG-58)
    • Wireman CQ127    (RG-58C)
    • Wireman CQ110    (RG-213)

  • Radio Shack
    • Tandy Cable RG-8X
    • Tandy Cable RG-58
    • Tandy Cable RG-59

  • Andrew
    • Andrew Heliax LDF4-50A
    • Andrew Heliax LDF5-50A
    • Andrew Heliax LDF6-50A

  • Wireman (ladder line)
    • Wireman 551 Ladder Line
    • Wireman 552 Ladder Line
    • Wireman 553 Ladder Line
    • Wireman 554 Ladder Line
    • Wireman 551 (wet)
    • Wireman 552 (wet)
    • Wireman 553 (wet)
    • Wireman 554 (wet)

  • Miscellaneous
    • Generic 300 ohm Tubular
    • Generic 450 ohm Window
    • Generic 600 ohm Open
    • Ideal (lossless) 50 ohm
    • Ideal (lossless) 75 ohm

The "wet" numbers represent worst case for lines covered with ice or snow.


Here's a quick line loss calculator to use  Note that the simple program used for this web page gives a very close approximation for additional losses due to SWR. 
Set Parameters as Desired
Line Type:
Line Length: Feet Meters
Frequency:  MHz
Load SWR:  : 1
Power In:  W
Results
Matched Loss:  dB
SWR Loss:  dB
Total Loss:  dB
Power Out:  W


Online Coax Cable Loss / Antenna Gain Calculator


Enter dB Loss Of Cable Per 100 Ft. At The Desired Operating Frequency

Enter Length of Cable in Feet

Enter Power into Cable in Watts

Enter Gain of Antenna in dBd

This program is provided "as-is". It is thought to be accurate but it is the responsibility of the user to verify the accuracy of the calculations when using this program.  

Return to Home Page


Formulas To Design Your Own Dipoles And Inverted Vees

INTRODUCTION-The longwire antenna is a very effective antenna for the listener who wants to cover all of the shortwave bands from 530 KHZ to 30 MHZ. However if you have some favourite frequencies that you listen to on a regular basis you may wish to consider a dipole antenna. This antenna is a fairly easy to construct antenna and will give you better reception on the frequency it is cut for. Think of a dipole as a longwire that has a insulator in the middle.

FREQUENCY-A dipole antenna will not only work well on the frequency it is cut for, but also for the multiples of that frequency. For example if you cut a dipole for 7.0 Mhz will also work well on 14 Mhz, 21 Mhz and 28 Mhz. This way if you can pick and choose your frequency you can make one antenna work on two or three bands.

LENGTH- To find out how long the antenna should be all you have to do is fill in a simple formula:

468 divided by FREQUENCY IN MHZ  = LENGTH IN FEET
300 divided by FREQUENCY IN MHZ  = LENGTH IN METERS (wavelength) 

168 divided by FREQUENCY IN MHZ =  DIPOLE LENGTH
(with end effect calculated in) 

That is the only formula you need ever know to build a dipole antenna.


This page uses the standard formula, 468 / f MHz to calculate dipole lengths. You may change this number if you know of a better number to use as your starting point.
Enter the formula for the antenna calculation

Divided by Freq MHz

Percent smaller for the Inverted Vee

Your dipole's total length is feet or meters
Each leg of the dipole is feet or meters
Your Inverted Vee's total length is feet or meters
Each leg of the Inverted Vee is feet or meters
 

Calculator purposes:  Each dipole leg length is a half wave leg

 

CONSTRUCTION- Once you have selected a frequency and calculated the length of wire you need add two feet to this length. This is done so you will have six (6) inches of wire at each end to wrap around the insulators. Once you have this extended length of wire cut it in half. This will give you both sides of the dipole.

Attach an end insulator to one end of each piece of wire. You can use the egg shaped insulators sold by many radio supply stores or make your own out of a piece of plastic. This can be done by cutting a piece of heavy plastic or plexi-glass to a size of about six (6) inches in length and about 2-3 inches wide. Drill a small hole one to one and a half inches from each end of the plastic to wrap the wire around. It is best to solder these connections and wrap them in a sealant tape to keep the effects of the weather from harming them.

The other free ends are attached to a center connector which you can buy with a built in coaxial cable connecter, or make your own. This will look similar to the end connectors but you will have to find a way to secure the coax lead wire to the insulator. If you build your own when you attach the coax to the ends of the wires, insure that you solder and wrap the connections. One wire will go to the center of the coax, while the other wire will go to the shielded braid of the coax. This will give you a perfect half wave dipole. You should also wrap the coax fitting of the commercially available center insulator to keep water and other moisture out. Moisture will ruin the connections on any type of insulator and make the antenna less effective or at worst useless.

MULTI-BAND DIPOLES- As was stated above you can use the dipole on the harmonics or multiples of the frequency it is cut for. However if you are short on space you can build a multi-band dipole. This way you will get an antenna that will operate on several frequencies. Instead of using a single strand of wire you can use wire that has several insulated wires in it. These MUST be insulated wires to insure that they do not touch each other. You then cut the top wire to be the longest, the second wire to be the second longest, the third wire to be the third longest etc.. Only the longest wire is attached to the end insulators and all wires are fed to the center insulator to attach to the coax feed line.

INSTALLATION- Once you have the antenna cut all you have to do in put it between two masts. Make sure that you use the free side of the end insulators to attach some rope. Tie this rope from the end insulators to the masts. Leave some slack on the antenna. If you pull too tight the antenna will break in the wind or if snow and or ice should coat the antenna. KEEP AWAY FROM OVERHEAD WIRES!! Keep away from these as should the antenna ever come into contact with an overhead wire you will do permanent damage to your radio if not yourself. All you have to do is feed the coax to your radio and listen to the stations come in. It would be best to install a lightening arrester in the coax feed line to help protect your receiver. These are available from many radio supply stores. Follow the instructions carefully! In areas where thunder storms or snow storms are common a lightening arrester is a must for safety.

You can install them flat or at an angle. If at an angle they will be more directional the direction that they are sloped. You can also install them as an inverted V shape. This dipole has a higher center with lower ends to save on space in smaller back yards. All three versions work well.

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Message #5582 of 6131 |
Expand Messages Author Sort by Date

Heya guys, Anybody got any bright ideas on how to reduce the signal coming out of the back of the transmitter? We have a one watt transmitter but wish to...
Michael Rowse
valdusradio
Offline Send Email
Feb 13, 2008
5:33 pm

Probably the cheapest/way to reduce power is to add attentuation to the aerial lead. Add length to the cable... not sure of exact figures but adding 10 metres...
Geoff Barkman
Mad_Milkie
Offline Send Email
Feb 13, 2008
8:13 pm

Geoff's right. The easiest way to reduce power is to use more coax. The data is in the back of the Dick Smith catalog. If I remember correctly the spec for...
John Peterson
johnfp3
Offline Send Email
Feb 13, 2008
8:40 pm

Speaking of 2.4GHz spectrum. I just hooked up some 600mW Wi-Fi PCI cards to have a look at the 2.4GHz spectrum in Invercargill. The amount of wireless LANs is...
Gavin Stephens
kiwi_rock_24
Offline Send Email
Feb 14, 2008
12:05 am

This is of interest to me, as I need to move my transmitter for a few weeks while decorating the studio. I have a 7 metre coax extension lead. Will this cause...
88.4 FM The Cheese
grantthoms
Offline Send Email
Feb 14, 2008
9:46 am

Hello Grant, Will be a little loss in TX power... now to get my calculator out :))) If your TX is out putting 500mw @ 88.4 MHZ, SWR of 1:1 at 7 meters of coax ...
Johnno
j_larsen2006
Offline Send Email
Feb 14, 2008
11:01 am

That's great Johnno.. Thanks for the info :) Cheers, Grant ... of coax...
grantthoms
Offline Send Email
Feb 15, 2008
9:22 am

Hey Johnno, Studying your figures, it seems that RG58A has a loss of 13mW per metre? So would I be right in saying to get a 1 watt TX down to approx. 300mW or...
Michael Rowse
valdusradio
Offline Send Email
Feb 14, 2008
5:57 pm

The figures are based on the Freq Range, SWR ratio (ie: 1:1) and the cable that is being used... If we use 88.4 mhz, 1 Watt, SWR 1:1 and using RG58A = 39...
Johnno
j_larsen2006
Offline Send Email
Feb 14, 2008
7:34 pm

I've been watching all the suggestions for reducing power and that is a simple and practical way of dealing with it. However if you have the schematic diagram...
averagesteward
Offline Send Email
Feb 14, 2008
7:40 pm

Unfortunately it's not as simple as working it out to a mW per meter. That's why things are done in dB. For example, driving a coax cable with a known loss of...
Gavin Stephens
kiwi_rock_24
Offline Send Email
Feb 15, 2008
1:13 am

You can make yourself a simple T attenuator with a loss of about 5.2dB Close to that value will suffice as its not exactly critical. Build it in a tin box with...
Keith
wantok.geo
Offline Send Email
Feb 15, 2008
1:25 am

By the way, it's 500mW eirp, output from the antenna... Cheers Dean Groove 107.7FM PO Box 10 989 Wellington Ph 381 4766 (381GROOVE) Fax 381 4755 Mob 021-457...
Groove 107.7FM
groove_crew
Offline Send Email
Feb 15, 2008
6:34 am

Thanks for those, Gavin. D. ... From: Gavin Stephens To: LPFM_Radio@... Sent: Friday, February 15, 2008 3:10 PM Subject: Re: [LPFM] Re:Reducing...
Groove 107.7FM
groove_crew
Offline Send Email
Feb 15, 2008
6:57 am

That coax cable one is handy but I've just finally noticed the power budget one isn't right. It asks for dBm on a lot of things and provides answers in dB when...
Gavin Stephens
kiwi_rock_24
Offline Send Email
Feb 15, 2008
11:42 am

... Reduce the drive to the final output - usually by reducing the voltage supplied to it. The final will last longer and run cooler....
wireless.radio
Offline Send Email
Feb 16, 2008
6:43 am

Hey guys, I just realised that I never actually thanked everyone for their ideas on reducing the signal output. Shame on me. So....thanks everyone! We have...
Michael Rowse
valdusradio
Offline Send Email
Mar 1, 2008
9:51 pm

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