Date: Wed, 13 Mar 2002 03:13:27 -0800 (PST)
From: "John Sexton" <computernetworks@go.com>
Subject: Re: LF: Re Antenna experiments
To: rsgb_lf_group@blacksheep.org
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Hi Jim,
Thanks for that clarification. It all helps to understand better what is going on.
I found the last bit especially illuminating. Since the first several metres of my antenna are sandwiched between the house and the steeply rising bank, a lot of the antenna can be thought of as being in a hole in the ground. This is probably the reason for the high ground loss. The answer appears to be to get the antenna up and out of this space.
I intend to experiment with raising the loading coil as high as is practicable to get the high voltage end and the first few metres of wire further away from the ground. I had previously found that moving the loading coil as far away as possible from surrounding objects considerably reduced the resistance at resonance.
At the same time I plan to increase the length of wire in the helix to raise the top capacity while keeping the capacity of the lower part of the antenna as low as possible.
The only other alternative is a Transmit Loop, but one step at a time.
BTW I have had some problems with burning at the top of the fibre glass mast and am wondering whether the filler might be carbon black??
73, John, G4CNN

-----Original Message-----
From: "James Moritz"<j.r.moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Tue Mar 12 11:31:16 PST 2002
Subject: LF: Re Antenna experiments (LONG...)

>Dear John, LF group>
>Some comments on John's queries about ERP and measuring antenna efficiency 
>- if of no interest, please delete now!
>
>The first thing is to be clear about what is meant by ERP. There are 
>different ways of expressing it, but what they amount to is that ERP is the 
>amount of power fed into an ideal reference antenna, in an ideal 
>environment and at the same distance from the receiver as the actual 
>antenna, that would produce the same field strength as the actual antenna. 
>Thus ERP is not really a measurement of power at all, but a measurement of 
>field strength with the implicit assumption that the field strength is 
>proportional to the square root of the transmitted power and inversely 
>proportional to distance. Knowing ERP tells you how strong the received 
>signal should be at a given distance, which is usually the most important 
>thing, but is not directly connected with the radiated power, or the 
>antenna that is radiating.
>
>The type of reference antenna is purely a matter of convenience - for 
>instance, an isotropic antenna does not exist, but it is easy to calculate 
>the field strength it would produce if it did, and therefore EIRP. A good 
>approximation to a dipole in free space is possible at UHF, and is a 
>convenient standard antenna, but of course at LF you can never get far 
>enough from the ground to get the same behaviour as in free space, even if 
>you could somehow put up the dipole. Nonetheless, you can still easily 
>calculate the power you would need to feed in to an orbiting LF dipole, to 
>get the same field strength in your fictional space ship 1 mile away, as 
>you are measuring while standing on the ground 1 mile from a real 
>transmitting antenna, and this would be the ERP (EDRP) of that particular 
>signal. This is relevant to us because the UK amateur radio regulations 
>define maximum power output on LF as ERP specifically with reference to a 
>dipole. A short, ideal  monopole over a perfect ground plane is probably 
>the most practical LF reference antenna - it does not exist either, but at 
>least over a limited range of distances, a very efficient, well sited 
>vertical ant produces something close to the same field strength and 
>directional pattern as the theoretical monopole would. Once again, the 
>important thing is that it is easy to calculate what field strength would 
>be produced by the theoretical reference antenna.
>
>For an isotropic antenna it is easy to work out the relation between 
>radiated power, distance, and field strength, since the power density is 
>uniform and independent of direction. The same amount of power applied to 
>an ideal dipole in free space would produce a greater field strength 
>because the dipole has directive gain; the ERP is defined in terms of field 
>strength produced in the direction of maximum radiation. It is quite 
>complicated to calculate the directivity of an antenna, but you can find 
>the details in any antenna theory text, and the directivity of a dipole is 
>about 1.67, whilst by definition directivity of an isotropic antenna is 1. 
>So for a given power, the dipole will give 1.67 times the power density, or 
>sqrt(1.67) times the E field strength. Or, for the same field strength, the 
>input power required is 1.67 times less than an isotropic antenna. 
>similarly, a short monopole with a perfect ground plane has a directivity 
>of 3. The formulas relating ERP, EIRP, and EMRP to field strength E and 
>distance r are:
>
>EIRP = (E*r)^2 /30;    E = sqrt(30*EIRP) /r
>
>ERP = (E*r)^2 /50,  =EIRP/1.67  E = 7*sqrt(ERP) /r
>
>EMRP = (E*r)^2 /90, =EIRP/3;   E = sqrt(90*EMRP) /r
>
>EIRP, ERP, and EMRP therefore differ more or less by the factors John gave, 
>which are the directivities of the different reference antennas. Once 
>again, it does not matter whether you use ERP, EIRP, or EMRP provided you 
>are consistent - since they are just measures of the field strength; 
>exactly how the field is generated is not important. So the type of 
>transmitting antenna actually in use, or how efficient it is, is not an issue
>
>The efficiency of an antenna is a bit harder to define. One definition 
>would be the ratio of power radiated to total power fed into the antenna. 
>This is the same as Rrad/(Rloss+Rrad), however while it is fairly easy to 
>measure Rloss+Rrad, the resistive part of the antenna impedance, it is not 
>possible to directly determine Rrad. One approach is to calculate Rrad, 
>using one of the formulas or numerical modelling techniques like NEC. 
>Calculating Rrad assumes a knowledge of the current distribution in the 
>antenna, and this depends on many unknowns for typical amateur antennas - 
>influence of trees, buildings, ground conductivity etc, etc. So in less 
>than ideal sites, the calculation is at best an approximation. If we assume 
>that the effective height of John's antenna is about 8m, we can calculate 
>Rrad = 160*pi^2*(h/lambda)^2, = 21milliohms.
>
>  Another approach is to infer a value of Prad or Rrad by measuring the 
>field strength and calculating the ERP. To get from this to the actual 
>power radiated by the antenna, you have to assume a value for the 
>directivity of the antenna, since the radiated power required to produce 
>this field strength depends on the directivity. The best assumption for a 
>tuned vertical antenna is that it has the same directivity as an ideal 
>monopole - practical experiments show this is fairly close to the truth. If 
>we then calculate the EMRP (not ERP or EIRP) from the field strength it is 
>the same as the radiated power from the antenna, Prad. Prad is the power 
>which we would need to feed into the antenna to get the measured field 
>strength if the antenna was 100% efficient.The radiation resistance can 
>then be calculated if the antenna current Iant is known; Rrad = 
>Prad/Iant^2. Or we could calculate the efficiency directly from the 
>transmitter power output, Ptx; Efficiency = Prad/Ptx. Then again, if we 
>know Ptx, Prad, and Rloss, Rrad is very nearly Rloss*Prad / Ptx. So in 
>John's case, 10uV/m at 63km gives an EMRP using the formula above of 4.4mW, 
>and the efficiency is 4.4mW / 200W = 0.0022%. Rloss is 93 ohms, so Rrad is 
>93ohms * 4.4mW/200W = 2milliohms.
>
>There is usually a difference between Rrad and Prad determined from field 
>strength measurements, and calculations from antenna geometry.The 
>difference between the radiated power calculated from what is going in to 
>the antenna, and that calculated from field strength measurements has been 
>called "site loss"; the site loss in John's case is about 10dB - quite high 
>compared to mine of around 3dB, but similar to G3XDV's antenna. Quite what 
>happens to the missing power is not clear - The argument is clouded also by 
>the many uncertainties and unknown factors. For example, it is difficult to 
>find out just how much antenna current is going down a nearby tree, and so 
>find out what effect that has on radiation. The field strength measurements 
>I did showed apparently random, but consistent variations of a few decibels 
>in field strength from one place to another. In my case, with site loss of 
>only a few dB, so it is hard to know if it is a real effect or not - 
>although the fact that most people measure lower field strengths than they 
>expect suggests that it does exist. But the practical upshot is that it all 
>adds to the difficulty of obtaining high ERP.
>
>Hope this is helpful and not too long - have now forgotten what I started 
>out to do!
>
>Cheers, Jim Moritz
>73 de M0BMU
>
>


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