230 ft isn't long enough to resonate on 80 meters. In a trapezoidal
shape it probably was resonant somewhere around 4.9 MHz. That means the
SWR on 80 was high, but with good 600 ohm line the feedline loss should
only be about 0.8 dB at 3.5 MHz. If you had fed it with 100 ft of
Wireman 551 ladderline, the feedline loss would have been about 2.7 dB.
Since the loop was so low, its gain should have been only been about 2.2
dBi (straight up). The gain at 10 degrees elevation was probably about
-13 dBi maximum.
If you feed a 130 ft dipole at 50 ft with good 600 ohm line, the gain
should be about 6.3 dBi straight up and about -5.6 dBi at 10 degrees
elevation over average ground. That's about 4 to 7 dB better than your
loop depending on elevation angle. If the feedline was not good
quality, then the difference could be much larger.
Your reasoning for the noise difference is probably correct.
Jerry, K4SAV
Rob Atkinson wrote:
> A few years ago I put up a 230 foot long horizontal loop fed with 600
> ohm ladder line. It was roughly in the shape of a quasi
> trapezoid-rectangle and was ~ 25 feet up. I used it mostly on 80 and
> 40 meters. I figured it was just barely high enough to avoid ground
> loss on 80 m. My noise level on that band was usually around S9 being
> in town on a small lot.
>
> A few weeks ago I took it down and put up a 130 foot long center fed
> dipole at 45 to 50 feet fed the same way. This has been blowing away
> the loop on 75 m. by 10 to 15 dB. The ground loss with the loop must
> have been much greater than I expected it to be, the lesson there
> being for me, that 25 feet is too low. A second comment is that the
> noise level on the dipole is about 2 S units less and more important,
> the RFI I used to get from local consumer appliances has vanished. My
> guess is that the dipole is a bit more isolated from the small fields
> these devices generate, by both being higher, and by not being near
> the homes where these devices are located (the loop, being a loop was
> near 3 homes). So, if you are in a city, consider that height may
> get you some distance from consumer electronics noise.
>
> Those of you concerned with common mode RF on coaxial feedlines but
> desire the advantages of balanced feedlines should consider a balanced
> link coupled tuner such as the Johnson Matchbox design since its
> inductive coupling isolates the unbalanced low Z line to the shack
> from common mode RF on the balanced feedline, without the use of any
> choke balun.
>
> 73
>
> Rob K5UJ
> _______________________________________________
>
>
>
>
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