Welcome to the LOOP Antenna Community!                                                                  Join and enjoy it!

We've been thinking of a community where we can all exchange ideas, opinions, or simply have a friendly chat about LOOP Antenna.

We rely on all members to help keep these discussion forums a safe place for people to share and view information.

So, please treat other community members the way you would like to be treated: be courteous and respectful, stay on topic, share your knowledge.

We will remove any post that includes personal attacks, swearing or offensive language, non-constructive criticism or falsehoods. Such a poor behavior will result in losing the community posting privileges.                                                                                                                                                                                         Thank you for your time and your help!

Martin Smith
mag 20

Midi loop narrow bandwidth

5 commenti

Hi folks, has anyone had a problem with the bandwidth being very narrow on 80m

i have checked the loop on an analyser and it only has a bandwidth of 1khz between the swr of 1.5 either side of the resonant point which in my case was 3.545mhz

i wonder what the plus and minus swr figures are when Ciro quotes 4khz bandwidth on 80m?

any help appreciated... please note, as I bought the loop second hand I have not used any extra compound on the joints... just a thought, but would it narrow the bandwidth that much?

mag 26
Migliore risposta

Hi, Martin,

The bandwidth of a tuned circuit is measured between the half-power points, and is related to the 'Q' of the tuned circuit. It is equal to Fr / Q. The quoted Q of the MIDI loop is 1,500 @ 3,500kHz, and therefore, assuming that there are no additional losses, this would make the bandwidth 3,500 / 1,500 = 2.33kHz. Ciro-Mazzoni also state that the bandwidth at 3,500kHz is 4kHz, which would imply a Q of 3,500 / 4 = 875. So there is some inconsistency here. In order to measure the bandwidth using an antenna analyser, if the SWR at resonance is essentially 1:1, then bandwidth is measured between those frequencies where the SWR is 2.62:1. If you have not used any additional conductive grease, then any increase in loop resistance would have the effect of increasing the bandwidth not narrowing it as any increase in loss resistance would lower the Q factor. Also bear in mind that most of the loss resistance of the loop is due to skin effect as RF currents only flow in the outermost section or 'skin' of the tubing, so conductive grease is most effective on the outer edges of the joints. I use an automotive product called Copper-Eze which is a grease that is loaded with micro-fine particles of copper and is used to lubricate disk brake pads to stop them squealing in their holders. In fact, any increase in apparent bandwidth is cause for concern, as it would indicate an increase in the loss resistance and a consequent serious reduction in efficiency. So, in your case, record the frequencies where the SWR is about 2.6: to 3:1, subtract one from the other, and that is your effective bandwidth. For the MIDI loop this is just about enough to accommodate a SSB channel.

mag 28

On 80m I tune the antenna 1kc below the QRG I will talk. This make the job fine.


73 Manfred, HB9FLU

Martin Smith
mag 28

Yes... I have been doing this as well. Definitely helps. Thanks

mag 28

Perhaps I can add a comment to explain why this might help. Bear in mind that the frequency displayed on your transceiver is the suppressed CARRIER, on 80m we use LSB, and thus the transmitted signal will be lower in frequency. If you are using a 2,400Hz bandwidth, from 300Hz to 2,700Hz, then the centre frequency will be 1,500Hz lower than the carrier frequency. Since the bandwidth of the MIDI loop on 80m is very narrow, of the order of 2kHz to 3 or 4kHz then tuning the antenna to a frequency between 1 and 2kHz lower than the displayed frequency ensures that all the transmitted power is placed in the centre of the antenna's effective bandwidth. This should only be necessary on 80M SSB. CW, and other narrow band transmission modes would not require this. Also, it should not be necessary on 40m, where the bandwidth is twice that of 80m.


73, Adrian, 5B4AIY

Martin Smith
mag 28

I was aware of the reason. Thank you.

I post più recenti
  • gm7eey
    un giorno fa

    Anyone suffer from poor tune on 20m when the Midi loop gets wet? When I build this one I noticed that the Gamma match tubing had a badly seated cap, when I removed it to inspect a few drops of water came out of the tube. Am going to take the loop down today and inspect this again as its been quite wet recently but am interested to know if this is a known issue. This is a used loop and as far as I know it was stored indoors prior to my assembly and when I did the build, the loop was only outdoors for about 2 days before it went on the mast. Was rather surprised to see the water and trying to tun last night when raining, 20m was nothing below 2.1, today now its dry kinda back to normal at 1.6 ish. Real concerned that water is getting into the co-ax section and thats bound to be bad news. 73s Pete
  • Robert Carswell
    ott 23

    Hi I have a brand new Baby Loop and have a problem straight away. After assembly I went through the installation First Turn On process. Set the loop type and moved to using the star key and setting the frequency as advised to 7050 pressing the enter key looks as though it will do something but the ATU just shuts down and restarts. I have attempted the Auto Detect routine and it fails with same issue the ATU just shuts down and restarts. The display shows 18000 khz with a SWR of 1.6 which is presumably after the factory tested the loop. The firmware is 3.29 and the board is 1.06.0663. I have downloaded the firmware again as per the manual and it doesn't appear to do any upgrading as presumably it already knows the firmware installed on the ATU is version 3.29 I have checked the twin core cable I am using with a 12volt battery and it checks out with the volt meter. I have checked the antenna cable which is mini 8 and that checks out. I have attempted the procedure for manual testing of the motor control as per the manual on page 70 entering the frequency of 7050 pressing enter which shows TEST: Duty 21% 7050khz SWR: 9.9 Pressing 9 to open the loop just shuts down and restarts the ATU During the manual testing I have checked across the motor cable plug on the rear of the ATU and there is no voltage at the terminals during any of the setup procedures or the manual testing of the motor control. This is a brand new never used before Baby Loop and fails at the first turn on stage. I have contacted the supplier but being the weekend there is no one available to help Any one any idea of the issue?
  • jeffsmithchicago
    set 19

    I received the Baby loop last week and assembled it. After setting it up a few feet away from the desk I decided to connect it and run some tests to give it a shake down. It worked perfectly considering it was only a few feet away from my radio desk and then it just seemed to die with no error messages or anything. The loop is "open" all the way and will not move at all. When manually entering a frequency it goes thru the normal message of tuning but does nothing else, no motor sound or anything. Any advice to get me going in the right direction?

Ciro Mazzoni Radiocomunicazioni


via Bonincontro, 18

37139 Verona -ITALIA-


Tel: +39 0458903104 Fax: +39 0458902633


P.IVA 03063020238

© 2014 created by Sergio Mazzoni