Re: Mobius Shielded Loop Antenna
- Hello Group
I just wanted to update you on the Mobius loop as described by Everett N8CNP . I finally got my RG/11 although it turned out to be a cable with aluminum shielding and some kind of lubricant between the shield and the jacket a real mess to work with. I used it anyway.
So I find that it performs much better than a single turn loop. In fact it equals or outperforms my reference antenna which is an 88 foot twinlead fed inverted Vee at 45 feet. But only in the evening. During the day the loop is almost useless. Have not figured that out yet.
One other thing I was hoping you might be able answer. The loop performance drops off considerably by the time I get to 40 meters. On 30 meters it is nearly deaf.
I am thinking there is a problem with my amp. Do you think T1 should be adjusted? Maybe I am not coupling enough high frequency voltage from T1? I was thinking maybe 15 turns is too much? Also, can I use a F T50 core, and if so, how would this effect the number of turns?
What do you think about the transistors? I get superb performance from 100 khz up to around 4 or 5 Mhz, but after that it really takes a dive. I have a pair of 2n5109's but have not installed them yet. Do you think it would be better than the 2n3904's I use now?
--- In email@example.com, everettsharp@... wrote:
> Hi Andrew,
> Thank you for your comments and you input. I tried several different hook
> ups with the standard shielded loop configuration and even tried a matching
> transformer using one of the 73 mix binocular cores, with several different
> ratios, but was never able to get it to work just right. I could not get
> the loop balanced correctly, as my 180 nulls were no where the same. Maybe
> I just did not come up with the correct combination.
> However, with the Mobius configuration it worked from the get go, with the
> M0YAF amp, of which I built 4 of them. I also tried the LZ1AQ broadband
> amp, but it did not work very well on any of the shielded loop configurations,
> but did okay on the standard 14 gage wire loop.
> Your observation on the lower performance above about 16 MHz is very
> similar to what I am seeing, but as I have moved up from first, from RG58, then
> to my home made coax and now to the RG11, I am seeing improvement. The RG 58
> has a capacitance of around 30 pf per foot, I did not measure my home made
> coax and the RG11 runs about 20 pf per foot, so again in your remarks on
> the capacitance maybe what is degrading my top end performance.
> 73 Everett
> In a message dated 8/19/2013 2:55:40 P.M. Central Daylight Time,
> andrew.ikin@... writes:
> Dear Everett,
> Thanks for sharing your observations. I would suggest that the difference
> you are seeing in loop performace comes down to how well the loops are
> matched to the ampfiliers and whether you are using a balanced amplifer to
> cancel out any near E field noise. All the mobius loop is doing is increasing
> the loops impedance. This could be have been achieved with a transformer.
> Some time ago I compared a Mobius loop (1m diam) to a 1m 3/4 tube loop
> using an ALA1530 Amplifier and a higher impedance low noise FET amplifier.
> What I found was; below approx. 16MHz both loop types would provide similar
> performance when taking into consideration the differing loop z to amplifier
> input z.
> Above 16MHz the capacitance of the Mobius loop was too high and thus the
> gain drops off. Where both loop types have a resonable match to the
> amplifier the signal and s/n are the same.
> If one looks at the two loop types intuatively, both have the same area,
> so it would be fair to say that they capture the same signal. With the
> mobuis the deliverable current to the amplifier is 4 x lower because the
> impedance is 4 x higher. Hence this is why Pixel use a 50 ohm input z amplifier
> to acheive good results compared to some designs using a near zero input
> impedance. The basic fallacy with using a near zero input z amplifier is
> that a loop is not a low z current source, but a current source with a series
> reactance. It is this rising reactance verses frequency which dominates
> the loss in a broadband loop antenna, hence the need to provide a reasonable
> loop/amplifier match.
> Andrew Ikin