```Dear LF Group, ```Thanks for the comments about field strengths, propagation modes, etc. I have done a bit more work on the subject: I have revised my estimates of the radiation resistance of my antenna, using EZNEC. If zero losses and a perfect ground are assumed, EZNEC gives a value for the radiation resistance of the antenna as 19 milliohms, as against the original estimate of 21milliohms, for my 8.1m high antenna. I also tried estimating the effects of objects near the antenna, ie. house and shed, by adding grounded wire boxes of aproximately the right size and shape. This reduced Rrad to 17milliohms. There don't seem to be any other objects nearby that could radically affect the radiation resistance. I also used G4FGQ's TANT136 program to estimate the radiation resistance. This yielded a value of 17milliohms also. Using this new value, and 1.4 A antenna current, radiated power comes to 33mW, and ERP is 60mW. The expected field strength at 4.2km is 408uV/m, against the measured value of 195uV/m. The discrepancy is reduced a bit, from 7.8 to 6.4dB. HB2ASB points out that field strengths are subject to quite large measurement errors. Measuring vertically polarised fields at LF is regarded as relatively reliable; from the equipment manuals, the loop antenna calibration is to within 1.8dB, and the analyser within 1.1dB. Allowing a further 1dB (about 10%) each for measuring the antenna current and dimensions, the total uncertainty is about 5dB. This is still not quite enough to account for the difference, and the errors are unlikely to all add together anyway, as the statisticians will tell you. Most Antenna text books contain graphical data on ground wave propagation. This is usually a graph of field strength vs. distance for various types of ground, normalised to 1kW power radiated from an electrically short vertical. Reading off at 4.2km, and scaling for 33mW radiated power gives about 400uV/m, the same estimated field strength as above - the ground losses are small over such a short distance. OH2LX also kindly sent me some computer calculated data, indicating that for 30mW radiated power, field strength at 4.0km should be 385uV/m, which is also close. The estimated field strength seems to be a good consensus. Of course, a single set of measurements is not enough to build elaborate theories around - what is needed are some more measurements, with different antennas and grounds, etc. Measuring over a short distance is important, because it effectively eliminates other propagation modes, such as ionospheric reflections. What use is all this? well, 2 things. First, it makes the field strength measurements made over larger distances much more useful - since you then have a good idea how much power was radiated in the first place, as well as how much arrived at the other end. Therefore, we can learn more about different types of propagation, effectiveness of antennas, and so on. The other important thing is to determine what ERP your station has - especially in countries where the 1W maximum ERP limit applies. The results I have obtained so far seem to show that calculated ERPs are rather over optimistic. A field strength measurement may be more realistic, and isn't very difficult to do (eg. see PA0SE's design for a field strength meter on G3YXM's web page - http://www.picks.force9.co.uk/pa0se.htm). The HP8591EM/HLA6120 measuring combination I used is rather deaf - the noise floor is about 26dBuV/m - and I'm not sure if the university would let me borrow it - which limits the possibilities for doing more measurements with it. G3XDV's signal could probably be measured from within the university though - how about it Mike? ```Cheers, Jim Moritz 73 de M0BMU ```