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Alex Loop (or other magnetic loop) Measurements


#1

Yesterday I made a summit-to-summit QSO with OK1DVM/P on 18 MHz. Miro was operating from OK/JC-040 using an FT-817 (5W) and Alex Loop antenna. Has anyone made any direct comparisons with other portable antennas such as the End-Fed Half-Wave (EFHW)?

73, Bill W4ZV (W4C/EM-047 on 2/3 Nov)


#2

In reply to W4ZV:

You can ask Mike KD9KC for his experiences as he uses an Alex Loop.

Andy
MM0FMF


#3

In reply to MM0FMF:
Hi Bill
Since 6 months I am running vy often a Alex-Loop. I worked N4EX and N1EU several times. I am vy happy with this antenna. QSY is very quick done!
Most of the time I get rst between 539-579 in DX and EU! Most RST are 559. Some hams always give 599 (hi)!
73 de HB9BIN (George)


#4

In reply to HB9BIN_3:
Hi all,

since June I am using a MFJ-1786 Magnetic loop on top of my roof, I am very happy with antenna.

Since I have my KX3 I work almost 100 % of time with it using just 5 or 10 watts.

Yesterday I worked W4ZV without problems, despite his signal was 519 but pretty audible.

I am thinking to get a 40m magnetic loop, only problem is the price.

Best 73 to all.

CT1DRB
David Quental


#5

In reply to CT1DRB:
Hi,

since several months I am using Alex loop with KX-3 on my SOTA
activities. I have used various antennas before. I never thought,
tha MLA is so efective, there is no problem to work EU and DX on
higher HF bands in particular.
First of all, there are very low demands for space and I am QRV
within 5 minutes after arriving to the summit.
My conclusion is that MLA performs well everywhere, the effect of
near environment is negligible in contrary to all the antennas I
have used before.

73’s Dan / OK1DIG


#6

In reply to W4ZV:
On tour with Herbert, OE9HRV on the summit Augstenberg HB0/LI-004. We compared the Alex-loop, 6mvertical(2radials6m) the pole is fixed at the cross and a 15m J-pole(L1/4 vertical). On 15m all 3 antennas showed about the same result!!! But the setup of the loop ant is the fastest.
I loaded a foto an Flickr

vy73, de arno oe9amj


#7

Wow, beautiful photo Arno! I worked Herbert and I wish I knew what antenna he was using at the time.

I’m going to remain skeptical on the Mag Loop performance until I can participate in one of the a/b tests myself, either on the receiving end or the transmitting end.

I wish there were better homebrewing plans available so I could quickly whip together a loop myself - I just find bits and pieces here and there but not a detailed cook’s recipe that includes source for the capacitor, etc.

73,
Barry N1EU


#8

In reply to N1EU:

Alex PY1AHD has been experimenting with these magnetic loops for a while. It is not difficult to build one from a copper tube or a coax cable. See http://www.alexloop.com/ . He has made also more light weight portable models. The weight is not so low though because you need to use large diameter copper pipe. The narrow band “high-Q” tuning improves both receiver sensitivity and transmission efficiency. The magnetic loop is worth of experimenting if you have the time for it.

73, Jaakko OH7BF/F5VGL


#9

In reply to OE9AMJ:
Hi Arno, glad to see you guys could finally make it up there - nice photo with the fog…
73, Matt/KØMOS


#10

In reply to W4ZV:
The Alex Loop antenna is a “small” antenna (electrically small when it is compared with the wavelength of the operating frequency). It has low efficiency and narrow bandwidth. The efficiency can be seen when you transmit. On receiving the AGC of the receiver compensates the low “gain” of antenna.
The End-Fed Half-Wave (EFHW)antenna is a full size antenna and has an efficiency accordingly.
Which one it is easier to mount, heavier, more portable depends by the experience, taste and so on.
In my opinion, if I would have to consider only the performance, than I would go for End-Fed Half-Wave (EFHW)antenna.
73 de Dan PA2LA


#11

In reply to W4ZV:

Hey Bill
I use a linked dipol with 50ohm koax(homemade), this splitting this and use a half dipol for the MP-1 (german HFP-1)!
Not problems with N1EU in the summer on 14mhz
I work only SSB!


#12

In reply to PA2LA:

Hi Dan,

It has
low efficiency and narrow bandwidth. The efficiency can be seen when
you transmit. On receiving the AGC of the receiver compensates the low

It is not the same thing like the ferrite rod antenna found from AM receivers. The efficiency depends on the total loss resistance for RF versus the radiation resistance of the antenna. The radiation resistance depends on the size of the antenna loop. It needs to have a reasonable diameter 1 - 3 meters and the loop needs to be made of 15 - 30 mm copper pipe to get the high Q-value around 1000. Or that was what I was calculating one evening. I did not build one yet.

Edit: the copper pipe is quite expensive in my local hardware store

73, Jaakko OH7BF/F5VGL


#13

In reply to F5VGL:

Hi Jaakko

I also intend to make up a loop to try sometime. While they are clearly not as effective as a full size antenna, they can be pretty useful on the higher bands when conditions are good. Silver plated coax cable might be worth investigating but the main problem is probably getting a really low loss system for the capacitor. Any sort of sliding contact system seems to be suboptimal.

I read somewhere that copper heating pipe is not quite as good as you might think as it has impurities in the copper that degrade the (RF) performance by increasing its resistance. I was intending to do the calculations to see if gold plating might be worthwhile. I need to work out the RF skin depth but I suspect that it is very small so the plating would not need to be very thick.

73

Richard
G3CWI


#14

In reply to G3CWI:

I need to
work out the RF skin depth but I suspect that it is very small so the
plating would not need to be very thick.

Yes it is small and most of the copper on the wall of the pipe is not needed for the RF conductivity. Thus in the classical magnetic loop design you pay for the extra copper, which adds also extra weight. Alex has done a loop from glass fiber PCB material. That might be better for a portable use with less weight. The PCB material is also practical for making the tuning capacitor. You need some mechanical rigidity for stable frequency tuning.

73, Jaakko OH7BF/F5VGL


#15

In reply to F5VGL:

I really need gold plated PVC tube.

73

Richard
G3CWI


#16

In reply to G3CWI:

I have found a supplier of 6mm o.d. 18ct gold tube. However a 2.5m length comes out at GBP16,129.32 inc VAT so I will be leaving that option alone.

73

Richard
G3CWI


#17

In reply to G3CWI:

Some Audio Note speaker cable is only GBP4800 per metre. That is solid silver so a lot of the metal isn’t needed really for RF. Still cheaper than gold plater tube.

Use a piece of LDF-250 or similar and just use the shield. It can carry serious RF currents so whilst less perfect than gold it will do OK.

Andy
MM0FMF


#18

In reply to G3CWI:

I need to work out the RF skin depth but I suspect that it is very small so the
plating would not need to be very thick.

73

Richard
G3CWI

Well clicking on one of my many saved links
http://chemandy.com/calculators/skin-effect-calculator.htm
it looks like you’ll need about 20 microns @ 14Mhz and you’re better off with Copper!

Pete


#19

In reply to G4ISJ:

How interesting. I have always thought that gold was a better conductor than silver and copper but its resistivity is higher. You have saved me a fortune; thanks!

73

Richard G3CWI


#20

In reply to G3CWI:

I think that in circuits gold is useful for contacts rather than conductors as it does not readily oxidize. The gold mating surfaces are therefore in excellent contact whereas with other metals some scraping away of the oxides is required during the connection process and only partial contact occurs.

For RF conduction copper coated steel tube might be expected to combine rigidity with excellent conduction in the copper “skin”, coating thickness presumably depending on both frequency and power handling. For a top quality connection the copper might be coated with gold at any points where mechanical contact rather than soldered contact was required.

I don’t remember whether silver is actually a better conductor than copper; I could look it up but it is getting late… Yes, it is, but not much.

73,
Rod