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Wire_saber
Junior Member
Username: Wire_saber
Post Number: 21
Registered: 4-2015
| | Posted on Tuesday, September 15, 2015 - 11:55 pm: | 
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Calculating the ERP of a station appears simple: take transmitter output in watts, subtract coax and other losses, and add antenna gain. Many companies help by disclosing the loss inserted by meters, antenna switches, tuners, etc. Andrews, Wireman & Times Microwave give us coax specs too.
While I do not seek absolute precision, I’ve had trouble finding catalogs or spec sheets to show loss inserted by different types of connectors such as elbow, male-male, barrels etc. Calculators I found in the web usually don’t list ‘alternative’ coax makers like JSC, Altelicon or ‘housebrand’ coax offered by many distributors, so I’ve searched such info separately. Fewer calculators allow input of VSWR as a variable. Inflated antenna gain figures are another point to contend (I wish all 14 dbd from my M-104c were real).
Do you know of an online calculator that would integrate all variables? Is there an easier or more efficient way to calculate (dbd) ERP? Are there generalizations that may be integrated for various connector losses? (I’ve read far-off suggestion to allow from .02 to .1 db per each). I know the ARRL book is a great source, but I guess I’m looking for the fountain of ERP youth!! |
Tech237
Moderator
Username: Tech237
Post Number: 1839
Registered: 4-2004
| | Posted on Thursday, September 17, 2015 - 10:52 am: |
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Best way, is to measure the power at the far end of you feed-line. This will include the losses for coax, joiners etc. Then calculate using the antenna "gain" factors. Keep in mind most manufacturers will exaggerate these a little.
If you really want to calculate it, then you may try creating a spreadsheet where you can enter all applicable variables and have it do the calcs.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Press_man
Advanced Member
Username: Press_man
Post Number: 881
Registered: 5-2008
| | Posted on Thursday, September 17, 2015 - 6:57 pm: |
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I was always told to guess at 1db per item thereby removing the panic of loss at unions. Look at the loss of a DF 3000 3 kw Low Pass filter. The manufacturer states-Insertion loss of < 1db.
I just try to get the best antenna/coax for the frequency and the best height for the surroundings and freq. As you said even the antenna is a variable. The 14db is more likely to be 12db. Like a PDL2 was rated at 12.5 I think it was. Even with it's beautiful design it was more like 8dbd.
73 Pressman
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Wire_saber
Junior Member
Username: Wire_saber
Post Number: 22
Registered: 4-2015
| | Posted on Friday, September 18, 2015 - 11:34 pm: |
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Press man: You have helped to 'debunk' the gain of the M104c. I found a less optimistic source that assessed the gain figure at 10 or 11 db max. The author was using the max gain that can be obtained from a 3-element beam, and then adding one or two db+ for the 'additional element' and boom length of an M104c.
Tech237: What a fantastic idea! Instead of guessing "other losses", just climb the tower, disconnect antenna and connect a watt meter with a dummy load at feed point. Take measurement after XYL key-up the transceiver, and add the antenna gain. To leave the antenna as the sole variable of the equation makes a lot of sense, and is the most accurate formula (barring meter variations & a badly-tuned antenna).
Wish I had posted my question earlier, as it would have saved me a lot of research time. Amplified thanks to Press Man and Tech237 for the great feedback. Time to unearth my boots & climbing belt! Will be comparing results to my previous 'theoretical' ERP calculations. |
Tech833
Moderator
Username: Tech833
Post Number: 2332
Registered: 8-2002
| | Posted on Friday, October 16, 2015 - 1:35 am: |
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The thing with loss is, actual calculations are still based in theory.
We assume that each coax connector is 0.1 dB of 'loss'. However, this also assumes that the connector is installed properly, is torqued properly, isn't cross-threaded, is clean, etc. You will NEVER be able to get a precise loss figure using theory, too many variables.
Best bet- Don't worry about your ERP. Unless you are trying to calculate ERP for path loss, who cares?
As for antenna gain, don't even get me started.
A good rule of thumb for yagi ('beam') type antennas is this- Every time you double the number of directors, you add 3 dB of gain.
For example, a 3 element yagi has about 8 dB gain. In order to jump to 11 dB gain, you need 4 elements. In order to jump to 14 dB gain, you now need 8 elements.
So, going from 3 to 4 elements (2 directors instead of 1), there is a sizable increase in gain. However, going from 4 to 5 or 6 elements, you would never notice it. Making the jump from 4 to 8 elements, you would notice that.
Your radio 'Mythbuster' since 1998
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