Topband: EIRP Measurement

W2XJ w2xj at
Tue Feb 21 11:32:49 PST 2012

The link provided below shows how to calculate EIRP. I think each 
administration will spell out the requirements of how to legally 
calculate EIRP. To me it is fairly straight forward. 60M in the US is an 
example. While there is an EIRP limitation, the FCC simplified the 
determination by assuming a dipole and the amount of power fed to it.

On 600M, one could easily put up as much tower as reasonable and then 
measure the tower's impedance. Once the impedance is known it is easy to 
calculate the input power to the radiator and therefore the current. 
This method is virtually universal for MW power measurement. There are  
calibrated RF ammeters available since they are required at each AM 
broadcast station (directional stations may use as many as a dozen 
depending on the array). Alternatively the voltage can be measured. In 
either case it is relatively easy to build and calibrate a voltage or 
current sampling device. That will establish the actual power input to 
the antenna. An 80 foot tower on 600M has around .6 ohms impedance 
therefore 1 amp will produce about 600 milliwatts into the antenna. 1.3 
amps would be 1 watt in this example. The antenna has 4.78 db gain over 
an isotropic radiator so this needs to be reduced to .75 amps which is 
340 milliwatts into the antenna (or 1.7 watts if a 5 W EIRP limit , 
slightly more than 1.5 Amps). This method eliminates having to calculate 
matching network losses as the ammeter is at the antenna feed point. I 
also think that actual field measurement is a bad idea and probably not 
allowed by most administrations. The reason is that the losses near the 
ground will be higher than the skywave component which is where there 
will be the possibility of interference. We will all have to wait and 
see how the rules are written.

On 2/21/12 12:09 PM, Brad Rehm wrote:
> I also think this topic is worth addressing, but Bill is probably
> right in saying it could gobble all the bandwidth on this reflector.
> Maybe someone would be willing to be the moderator of a new reflector.
> In the mean time, we should acknowledge that a lot of the work has
> already been done by the hams who've been experimenting on 136 kHz.
> See for example.
> BTW, the three-antenna method is not considered the best way to make
> field strength measurements at these frequencies.  Since the field is
> magnetic, loops, monopoles, helmholz coils, and other kinds of probes
> are normally used.
> I wonder how many 160 ops are considering trying the new band.  I
> wouldn't be surprised if the answer were "very few."  Although the new
> band is just below AM broadcast frequencies, instead of just above
> them, this is the region in which transmitters and antennas begin to
> look very different from conventional HF setups.  After we've made
> substantial investments in time and resources in decent 160m stations,
> moving down will present new challenges which may only interest
> "experimenters."
> Brad
> KV5V
> On Tue, Feb 21, 2012 at 8:49 AM, Bill Cromwell<wrcromwell at>  wrote:
>> On Mon, 2012-02-20 at 19:04 -0700, Tod - ID wrote:
>>> In thinking about the measurement of EIRP for the new band it occurred to me that this might be the time for a few folks to think about how we could fabricate a "commonly calibrated" field strength meter. Such a device might well be used on both 160m and the 600 kHz band. By commonly calibrated in mean a device that has an agreed upon standard way to calibrate it that can be done with simple methods. One thing that always intrigued me about the HeathKit products was the fact that they used very simple methods to allow the builder to adjust the device without expensive equipment. Sometimes that meant a special circuit built into the product which was only used for adjusting the device.
>>> The important thing is to have a way to assure that when someone measured the same field at the same point with the same type of measurement device they would get the same measurement result. That would allow us to compare measurements between different people even if we did not know the absolute field strength value.
>>> An interesting idea might be to make such a field strength meter have an interface to Wi-Fi so that it could be positioned at a remote point and send its readings to one's local network. There are a lot of ways to do that of course, but planning to have it part the device initially would simplify taking measurements. I am sure that there are others who could contribute ideas that eventually could be used to formulate design objectives for such a device. If there was enough interest I can imagine that a single PCB could created and a lot of such devices built to allow us to add to our collection of measurement devices that help us fabricate better antennas and stations.
>>> I wonder if I am the only one who thinks this might be a useful TopBand subject.
>>> Tod, K0TO
>>> _______________________________________________
>>> UR RST IS ... ... ..9 QSB QSB - hw? BK
>> I think this thread will quickly get of hand even though it may be
>> applicable on Top Band (and every other band). I took a quick look at
>> the lists and I didn't see anything resembling
>> "instrument(ation)" or "measurement". Are there lists or web groups for
>> amateur instrumentation? I will expand my own search and report back
>> here if I find anything useful but maybe some list member(s) already
>> know where to look.
>> The whole point of having standards is so that we can get the same
>> results under the same conditions as other people. It should not be any
>> more difficult to 'calibrate' to an existing, published standard than
>> 'calibrating' to a new 'nonstandard'. There are a lot of different
>> approaches to accomplishing that and we should look for them.
>> If there is interest we might start a new list to explore the
>> possibilities.
>> 73,
>> Bill  KU8H
>> _______________________________________________
>> UR RST IS ... ... ..9 QSB QSB - hw? BK
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK

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