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Calculating dB, dBi

n4jnw

n4jnw

Sterile
May 15, 2010
33
0
16
40
Irvine, Kentucky
Ok, so I've been sitting for the past few minutes crunching some numbers trying to come up with a figure for the ERP of my "10M ham" station now, and how the ERP will change when I re-do most of the coax run in a few weeks. I'm unsure how accurate my math is, so for those who actually KNOW how to do this, please chime in and offer up your corrections/advice.

This is a rundown of my station now, as it sits. Listing each item and the approximate dB loss for each item.

10 coax connectors, -5dB
1 coax switch box, -1dB
1 low pass filter, -1dB
75' RG8X coax, -1.2dB
1 wattmerer, -1dB

Total of 9.2dB loss from the radio to the antenna. At a 9.2 dB loss, a 50W "10 Meter" radio should have an ERP of 17W getting TO the antenna.

The antenna is a Solarcon A99, which reportedly has a gain of 1.8dBi, and I'm calculating roughly a 1.6X increase in power due to the gain. Which should then put the ERP actually being transmitted at 27W.


Now, fast forward to my new setup. In which I'm upgrading coax, and taking out an item or two.

6 coax connectors, -3dB
1 switchbox, -1dB
100' LMR400 coax, -0.64dB
1 wattmeter, -1dB

New total of 5.64 dB loss (A 3.5 dB change over the old setup) from the radio to antenna. At 5.64 dB loss, a 50W "10 meter radio" should have an ERP of 27W getting TO the antenna.

Again, Solarcon A99, 1.8dBi gain would yield an ERP of 43W actually being transmitted. If I upgrade to an Imax 2000 which has a 2.9dBi gain, my transmitted ERP should go up to 54W.

So, in retrospect by changing coax and removing a few coax ends and a low pass filter, I'm gaining 3.5dB over my current "old" setup.

Does everything gee and haw?
 

I haven't gone over the math but have big questions about the numbers being quoted. Where are you getting your gain and loss figures?

0.5dB per connector is excessive (even for a PL259). Changing to just 0.1dB per connector is likely high at 27MHz. 1dB for each of the switch box, LPF, and wattmeter are way too high. Just those three alone would eliminate half your transmitter power! Without accurate figures everything you say and any conclusions being made are meaningless.

Power gain = 10 log (P1/P2)

Every 3dB is essentially a doubling of power. -3dB is half power. If your system had 9dB loss, the output power would be 1/8 or 12.5% of what went in!

In practice, even 3dB is often not noticeable to a distant receiver. You could double (or half) your transmit power with little or no difference. Try it out for yourself if you have an accurate way to change transmit power by 3dB or if you have a good 3dB attenuator to install on your antenna receive line.

The only figures you've quoted that look reasonable are for the feedline loss. I haven't confirmed the numbers and will assume them to be accurate. The loss difference between your two feedlines is 0.56dB. Anyone who can measure that difference after the signal leaves the antenna has too much money tied up in test equipment. I've used $50k spectrum analyzers that will not resolve to that level on a receive signal. Unless the cable is damaged, I'd suggest putting your money elsewhere.

ERP is a calculated value based on overall system gain / loss. It can not be measured. You don't have 25 watts ERP into an antenna. You could calculate 25 watts arriving at an antenna after losses are calculated, or you could calculate 25 watts ERP after factoring in antenna gain with system losses and transmitter output power.

You mentioned 50 watts with 5.64dB loss. Let's assume it's 6dB instead. You will never notice the 0.36dB difference. The output power would be about 1/4 or 12.5 watts.

A more exact calculation would be 50 / 10^0.564 = 13.64 watts

Here's a good site to read on decibels.
http://www.dxing.com/tnotes/tnote10.pdf

Wikipedia is your friend too.
 
Last edited:
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now , if you replaced that 99 with a omni with lower lobes to put more signal where you are talking , that might more than make up for all your little losses ..... and put you ahead . BTW are you running a amp ? if you are , what is in line other than coax between the amp and the antenna ?
 
I also think that the connector loss is high: It should be around –0.1 dB, but I’ll concede to make it –0.2 dB to be conservative. The same for the Low Pass filter and the switches: They should also be about –0.1 each, but again I’ll be conservative and use –0.2 dB for each.

The antenna is another thing too. You’re quoting the Solarcon A99 at +1.8 dBi, that’s equal to –0.35 dBd, (1.8 dBi – (+2.15 dBi)). Remember 2.15 dBi = 0 dBd.

Thus for system 1:

50 Watts = 46.9897 dBm
LP Filter = -0.2000 dB
10 Connectors = -2.0 dB
1 Coax Switch = -0.2 dB
1 Wattmeter = -0.2 dB
75 ft RG-8/X = -1.875 dB
Solarcon A99 = -0.35 dBd

For this setup: Total Gains = 46.9897 dB
Total Loss = -4.8250 dB

ERP = 42.1647 dBm = 16.4615 Watts

Whereas:
EiRP = 42.1647 dBm + 2.15 dBi = 44.3147 dBi = 27.0066 Watts
-------------------------------------------------------------

For system 2:

50 Watts = 46.9897 dBm
LP Filter = -0.0000 dB
6 Connectors = -1.20 dB
1 Coax Switch = -0.2 dB
1 Wattmeter = -0.2 dB
100 ft LMR400 = -0.7 dB
Solarcon A99 = -0.35 dBd

For this setup: Total Gains = 46.9897 dB
Total Loss = -2.65 dB

ERP = 44.3397 dBm = 27.1625 Watt

Whereas:
EiRP = 44.3397 + 2.15 dBi = 46.4897 dBi = 44.5625 Watts

73,
Mike
 
BOOTY MONSTER said:
now , if you replaced that 99 with a omni with lower lobes to put more signal where you are talking , that might more than make up for all your little losses ..... and put you ahead . BTW are you running a amp ? if you are , what is in line other than coax between the amp and the antenna ?
Click to expand...

No amp. Strictly radio.
 
KC9Q said:
...The antenna is another thing too. You’re quoting the Solarcon A99 at +1.8 dBi, that’s equal to –0.35 dBd, (1.8 dBi – (+2.15 dBi)). Remember 2.15 dBi = 0 dBd.73,Mike
Click to expand...


after the incorrect high connector loss, ............that also was the first thing i saw too. the A99 actually has a negative DBd gain:whistle:
 
In summary; the changes you proposed will make not one bit of measurable difference. You would see more benefit to raising the antenna another 10 ft or adding an amplifier.
 
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BOOTY MONSTER said:
so it would take 100 properly soldered pl-259s to equal 1 db of loss ?
Click to expand...

Well look at it this way. Running 1500 watts through a connector at .1dB attenuation would be something in the order of 35 watts needing to be dissipated. That's enough to melt the solder.


Sent from my iPhone using Tapatalk
 
if 1500 watts will melt solder does that mean the folks running that much or more are using solderless connectors ??
 
Your not following the logic, the connectors are actually less than .01dB of insertion loss.


Sent from my iPhone using Tapatalk
 

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