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Home Brew Broadband Magnetic Loop

KA7UKA7U
edited January 2018 in Installation, Antennas and Interference
This LNA was discussed in an earlier thread and I decided to see how it might work with a home brew loop. https://www.amazon.com/gp/product/B01N2NJSGV/ref=oh_aui_detailpage_o00_s01?ie=UTF8&psc=1

I spent a total of $25.88 for parts. I had the PVC pipe for support and coax cable in the junk box. This little LNA connected to a loop made from 14' of coax and powered with a 9vdc battery is providing signals from 5KHz to ???, well at least 30MHz for the KiwiSDR. It has been tested, but is not currently online as I'm working to "ruggedize" the LNA in a small pipe.

I think an inexpensive antenna such as this could improve some KiwiSDR installations. It isn't perfect and displays more repeating interference spikes than I would like, but it is still better than many other antennas I've listened to. Not quite as good as the W6LVP but it is better below 500KHz and probably equal up to 18MHz. It needs more comparison testing and the loop circumference might make a difference.
Ron
KA7U



Attachments:
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Comments

  • KA7UKA7U
    edited January 2018
    I tested this loop quite a bit for the last two days. It is not on par with W6LVP loop, but it does work better than my miniwhip and other small antennas that I've tried. So on a scale of 1 to 10, with 10 being my 210' top dipole, the W6LVP rates a 8 and the home made loop with the HiLetgo amp rates a 4. What we need is a Low Noise Amplifier made specifically for broadband loops at a price point similar to the HiLetgo LNA.
    Ron
    KA7U
  • KA7UKA7U
    So today I changed the power to the LNA. I found that 9.25vdc and 10.5vdc results in the lowest SNR. At 60KHz 10.5vdc is optimum and at 15MHz 9.25vdc is optimum. 10.5vdc seems to be the good voltage overall if you want good results at VLF. This might vary for different LNA boards, but I doubt it. I used a variac to change the voltage on the wallwart to find the best voltage.

    This antenna is producing lots of interference spikes. Remove the antenna, the spikes go away. Remove the amplifier but not the antenna and the spikes still appear at a lower level. All the signals are still available just at a lower level. Add the LNA and the signals stand up and are easily received by the KiwiSDR over 100' of coax. This is more subjective than objective, but it seems to me that once the voltage input is optimized for 10.5vdc, this homemade antenna is about 2-4 dB less SNR than the W6LVP loop. So the W6LVP loop is the winner, but not by a huge SNR margin.

    I have not provided a balun to feed the amplifier and I have not followed all the "Tips", (only the voltage observation), provided by the seller, which follows:
    Tips:
    1.The amplifier at operating frequencies less than 500MHz, has a very good gain flatness, careful adjustment can be 1dB lower frequency, the higher the gain consistency
    2.The lower limit of the operating frequency of the amplifier is subject to the input and output capacitors, the default is 0.1uF, can work to 0.1MHz; can be appropriate to increase the input and output capacitors to expand the lower frequency limit,Such as: 10uF, you can work to 5KHz. For detailed calculations, pay attention to the input and output impedance of 50 ohms, calculate the required capacitance value, while enhancing power supply decoupling and voltage regulation.
    3.When the supply voltage varies from 5-8V, it can be used as a variable gain amplifier. The gain increases with the supply voltage. Ideal for RF receive front-end circuits, with DA control supply voltage, thereby controlling the amplifier gain, automatic gain control. When the supply voltage at 8-10V, the low-frequency gain of 30dB, this amplifier has a lower noise figure and better stability. 

    I might eventually change the input and output caps, which are surface mount and I have to order in, and possibly add in a balun to feed the amp, although I doubt it will add value.
    Ron
    KA7U

  • KA7UKA7U
    Since JKS organized the antenna and interference related threads to their own section, I found that JKS had posted this link:

    Reading that and noticing that my home brew loop is showing these spurs at a high level is spurring me to action. I'm planning to upgrade my network to shielded cable and switches and possibly relocating the KiwiSDR antenna which is most affected farther away from the shack. 

    Now back to why would the home brew loop produce higher amplitude spurs than the W6LVP loop? I am thinking it might be because the electrical characteristic of a shielded magnetic loop ( one end has shield and center conductor both common to the ground, and the other end uses the center conductor only to the input of the amplifier) should be an unbalanced antenna. The HiLetgo LNA is an unbalanced amplifier, the net result might be less cancellation of the 60KHz network spurs on the coax feedline?  The W6LVP loop amplifier is potted and a schematic is not provided but, George Smart https://www.george-smart.co.uk/projects/wellgood_loop/ , has a schematic of balanced LNA and the input to that is a tube which is a balanced input. The attached image shows that George Smart's KiwiSDR does have 60KHz spurs displayed at various amplitude, but they are on the order of magnitude as the ones produced by the W6LVP loop, which causes me to think a balanced LNA and a balanced loop will not be as strongly affected by the network spurs.

    I'll follow up on this topic after I upgrade the local network here at KA7U.
    Ron
    KA7U


    Attachments:
    https://forum.kiwisdr.com/uploads/Uploader/09/f3461fc0fabf669a8e0089b6f54d8b.png
  • G8JNJG8JNJ
    Hi Ron,

    I think that the problem is almost certainly loop imbalance. 

    You can check this by rotating the loop to peak a known station. Note the signal level then rotate the loop 180 degrees and note the signal level of the peak in that direction. 

    If the loop is properly balanced the signal levels should be the same. If it isn't the peaks will be different as the imbalance means that it is now configured as an Aperiodic loop. 


    Which are often used for phased receive arrays like this commercial version.


    I found that the LZ1AQ design was easier to build than the 'Wellgood' (Wellbrook copy) and performed slightly better.


    I fed mine with coax and left out the input filter network and voltage regulator (works OK on 13.8v) 

    The Wellgood and ZL1AQ work OK up to about 10MHz then run out of steam. if you want better performance on the HF bands the only solution is to make a 'fatter' loop or connect multiple loops in parallel in order to reduce the loop inductance. 



    More stuff about loops etc. on my active antennas webpage http://www.g8jnj.net/activeantennas.htm

    Regards,

    Martin - G8JNJ



    KA7U
  • KA7UKA7U
    edited January 2018
    Martin,
    Now that I've read through the information you provided and even more that Google was listing, I think I'm getting the scope of loop antennas. Hi Hi. 

    The Hiletgo amplifier is actually doing a reasonable job on the home brew shielded magnetic coax loop that I've been experimenting with. The gain is more than the W6LVP loop but it does not have as good a noise figure. I find that if I open the top of the loop and cut the shield and center conductor, cut coax in half, and then solder the center conductor which becomes the LNA input to the shield which becomes the LNA ground on the opposite side of the loop, make the shield continuous at the bottom of the loop, then the loop quiets down and the various spurs and images disappear. I think a better way to feed the Hiletgo LNA would be to make a loop out of copper tubing, place a T in the bottom of this loop and then cut out 2 or 3 inches from the top of the loop. feed a coax inside the tubing, solder the braid to one side of the tube and the center conductor to the other side of the tube and then connect that coax to the Hiletgo amplifier. I might give that a try using 1" copper water pipe. 

    I've ordered 3 Wellgood boards from Mr. Smart and the parts from Mouser Electronics. After those are assembled and working I'll report back. 

    My quest for an inexpensive loop antenna is getting expensive.
    Ron
    KA7U
  • KA7UKA7U
    The home brew loop with the Hiletgo amplifier is finally working adequately. So here is the setup that finally put it into respectable service. The idea that a working amplified loop antenna can be made with retail parts for under $50 is now demonstrated.
    • 1 meter diameter coax loop.
    • coax is cut in half and the center conductor that will feed the input to the Hiletgo amp is soldered to the braid of the half that will connect to the ground of the Hiletgo amp.
    • The center conductor on the side that is to be ground is soldered to the coax braid at the point it connects to the Hiletgo amp.
    • The center conductor that connects to the input of the Hiletgo amp is not connected to the braid on the coax that shields it in the coax.
    • The braids of both coax sides are soldered together at the feed point of the Hiletgo amp.
    • The power supply is critical and I now find 12.13vdc is optimum and needs to be a clean DC source.
    • check the attached images for better clarity.
    • The screenshot of a waterfall on the left is a 210' top dipole without an amplifier, the waterfall on the right is the shielded magnetic loop with the Hiletgo LNA. The settings for waterfall gain are the same for both displays.
    I'll leave it in service at http://ka7u.no-ip.org:8073 if you have an interest, try it out. 


    Attachments:
    https://forum.kiwisdr.com/uploads/Uploader/64/a4562c234d94356fc513b8712d2aec.png
    https://forum.kiwisdr.com/uploads/Uploader/95/557230ee784b4c86cb0fb8534471cb.jpg
    https://forum.kiwisdr.com/uploads/Uploader/f5/0ccbe595d6eb7a21794036089a9bfb.jpg
    https://forum.kiwisdr.com/uploads/Uploader/0a/78b245d2b9754f7500133e3a24b73e.jpg
    https://forum.kiwisdr.com/uploads/Uploader/32/aeb784b6fed9372a04f8f1cd850903.jpg
  • KA7UKA7U
    Here is a schematic of the loop. Please forgive my artwork.
    Ron
    KA7U


    Attachments:
    https://forum.kiwisdr.com/uploads/Uploader/c1/3eadf23364737dbc0d939a709553e4.jpg
  • KA7UKA7U
    A note about the split in the shield at the top of the loop antenna shown in the illustration above. The center conductor can be left continuous. It does not seem to make much difference in the antenna if it is connected to the shield or continues around the loop without a cross connection.
    Ron
    KA7U
  • G8JNJG8JNJ
    Hi Ron,

    I think the loop is still unbalanced.

    The split is to avoid a continuous 'screen' (Faraday screen to minimise E-Field pickup) around the loop. But if one end of the center conductor is connected to ground you may have a problem.

    Check signal levels with the loop normal and 180 degrees as suggested previously.

    A 1:1 transformer on the loop input may help improve balance. Try six bifilar turns through a BN73-202 core as a starting point. One winding as the (balanced) loop input (primary) the other as the output (secondary) unbalanced connected to the amplifier input, with one end of the winding connected to ground. This will also provide galvanic isolation.

    You can also try a few more turns on the secondary as an experiment as this may improve reception on the LF bands.

    If you made a balanced loop amplifier with a much lower value of input impedance (LZ1AQ or Wellbrook copy) you wouldn't need the loop outer screen to minimise E-Field pickup.

    Regards,

    Martin - G8JNJ
    KA7U
  • KA7UKA7U
    Martin,
    I believe the loop is unbalanced as currently connected. My next iteration will feature a balun to feed the LNA, the center conductor won't be grounded, and I'll check to see if it has a null in the axle directions.  My parts from Mouser Electronics have arrived, and the three Wellgood boards are in the mail and I'm looking forward to them arriving. Mr. Smart is sending his Bias-T boards as well, although I haven't yet ordered parts for those boards, when I do I'll order in several BN73-202 ferrites. Mouser has a 5 week delivery on them, but I do find them for twice the price elsewhere.

    My goal on this loop discussion is to build it for under $50.00/35.40£.  I'm at $42 now, so there is room for a balun. Hi Hi The Wellgood boards with the Bias-T boards and parts will be under $50 as well, as long as a few are ordered to reduce the postage, or a batch of boards could be locally etched I suppose.
    Ron
    KA7U


  • WA2ZKDWA2ZKD
    Related stuff here. I imagine the low end of that balun doesn't do well at LW but the board could be the base for alternate toroids


    KA7U
  • KA7UKA7U
    This photo is the cleaned up version of the loop being discussed. The 1/2" PVC pipe is not glued, relying on compression to hold together. This makes it easy to break down and use portable. The Hiletgo LNA works fine with a 9vdc battery.
    Ron
    KA7U


    Attachments:
    https://forum.kiwisdr.com/uploads/Uploader/af/0d97ab599078e1920c2db8f9b82ed5.jpg
  • KA7UKA7U
    edited February 2018
    It occurred to me to check the relative antenna / LNA sensitivity this afternoon. I connected a 50 ohm load to the transmitter and set it for 5 watt carrier output. Transmitter is 100' from the loop antenna. The loop and Hiletgo amp provided these results into the KiwiSDR. The second column of numbers is a commercial loop for relative comparison. Remember the numbers are negative so the smaller number is more gain, the larger number is less gain. The smaller the noise floor "NF" number the better, the higher the NF number the worse.
                          Hiletgo       W6LVP
    28.5MHz    -   -69dBm       -60dBm   note: The average NF on the Hiletgo is -122dBm, the average NF on the W6LVP is -115
    24.9MHz    -   -63dBm       -70dBm
    21.2MHz    -   -48dBm       -50dBm
    18.1MHz    -   -44dBm       -44dBm
    14.2MHz    -   -54dBm       -55dBm
    10.12MHz  -   -53dBm       -60dBm
    7.15MHz    -   -57dBm       -75dBm
    5.33MHz    -   -59dBm       -76dBm
    3.75MHz    -   -67dBm       -77dBm
    1.9MHz      -   -90dBm       -90dBm
    WWVB       -   -98dBm      -94dBm   note: NF Hiletgo -127, NF W6LVP -122

    The Hiletgo antenna impedance measures low < 50ohms on 21.2 and 18.1MHz. This would make me think the Hiletgo does better with a low antenna impedance.This antenna is dropping dramatically at 1.9MHz which is the lowest transmitted signal used. The antenna does receive the MF broadcast band quite well and so it might be a spot in the system that attenuates more than others or lower radiated power from the 50ohm load resistor and cables.  The W6LVP output impedance over the band has not yet been measured because the antenna is connected and potted and there is not a practical way to remove the LNA.
    Ron
    KA7U
  • KA7UKA7U
    Martin,
    I tested this Hiletgo loop for balance tonight and it is balanced. My test was with RNZI @ 15720KHz so it was skywave and not totally reliable, but the null on either side was deep and the gain off both ends was pretty much equal. So at least on this signal the antenna exhibits balance. I need to buy a rotator so I don't have to run the stairs. Hi Hi
    Ron
    KA7U
  • KA7UKA7U
    edited February 2018
    Martin,
    I wound a 7 turn bifilar on a bn-73-202 core and connected yellow to yellow on the antenna, and red to red on the Hiletgo LNA (red and yellow being the wire colors). I'm seeing between 5 and 10dBm gain with the Hiletgo LNA versus the good Wellgood balanced LNA. There is a static buildup that discharges with significant consequence when the wind blows across the antenna. I speculate that is caused because the loop is not directly DC grounded. I know you have significant experience with baluns and transformers so, what should I do different? Should I use an auto-transformer? Or should I go back to my original design with the loop split at the top, and the shields bonded and grounded with the inside wire also grounded?  https://goo.gl/JHqTe7  Now that I'm into this, I find another Ham that is following that design.

    I'm thinking an auto-transformer might be the cleanest and simplest way to do this, but the static has to be drained off somehow.  I'm not sure what is happening with the antenna balance, but suspect that if a transformer is used, the balance should be good.
    Ron
    KA7U
  • G8JNJG8JNJ
    Hi Ron,

    I think you could either center tap the input transformer on the loop side (prefered method), or add two 1K Ohm value resistors (or similar) from either side of the loop / transformer connection to ground.

    Regards,

    Martin - G8JNJ
    KA7U
  • KA7UKA7U
    Martin,
    Thank you for bearing me on this quest. I'm a bit compulsive. So I've now tried several balun types with the Hiletgo and while it achieves good gain the noise floor and images are over the top unless it is connected as a shielded magnetic loop, shield split at the top and shield and one side of the inner conductor grounded at the bottom on one side and the other side of the inner conductor used to feed the Hiletgo LNA. Then the noise floor and images drop down to an acceptable level. So my conclusion is to use a balanced LNA with a simple loop and a shielded magnetic configuration with an unbalanced LNA.

    While I've been experimenting I've learned that the CPVC pipe fittings are not going to hold up with the heavy copper loop, so I'll need to find a more secure support system for the Wellgood loops. PVC works great with a loop made from coax but then there isn't any stress with that material.  

    I'm not ready to post my conclusions about this loop comparison project.., 

    but I'm starting to think about saying that the Hiletgo LNA used with a 1 meter diameter coax loop, shield continuously bonded except at the top of the loop where the shield is opened, the feed point at the bottom of the loop with the shield and one side of the center conductor connected to the ground side of the Hiletgo LNA and the opposite center conductor connected to the Hiletgo LNA input, is a low cost, low stress way to get a decent loop antenna on the air.

    The Wellgood LNA while exhibiting balance, low noise, good feedline isolation, and good gain, is an expensive and stressful design to make. If you are not equipped with a good soldering station complete with a desoldering tool that works well the frustration might significantly increase. I say this because I have one successful build and one sub-par build and tomorrow the suspect transformer will have 8 leads desoldered and the core rewound and reinstalled. This will add significant time to the hobby project. Fortunately my friend K7RHB will do the desoldering work for me.

    The 1" (25.4 mm) copper tubing formed into a 1.2 meter loop adds quite a bit of gain to the loop antenna. I suppose this to be the result of increased loop area and reduced loop inductance. Besides PA3GZK claims 1.2 meter diameter is optimum in practice and who am I to doubt that? If I use the PA3GZK construction technique and use brass fittings instead of CPVC, a sturdy self supporting antenna base will result so add the significant cost of those fittings.

    Long story short, if you build a Shielded Magnetic Loop out of coax and use the Hiletgo LNA, reception should be good if not excellent depending on the coax diameter.  I'm still considering all of the above however.
    Ron Morell
    KA7U
  • G8JNJG8JNJ
    Hi Ron,

    I find your comment "Then the noise floor and images drop down to an acceptable level." interesting.

    I suspect that the wide bandwidth of the Hiletgo LNA may be a problem and that it is VHF and UHF broadcast stations that are overloading the amplifier.

    It's not easy to filter these out without adding additional unwanted inductance into the input of the amplifier stage.

    I would take a look at the filter on the input of the LZ1AQ loop amplifier. Although this messes up the LZ1AQ loop performance, it may work OK with the Hiletgo LNA as the input impedance is much higher than the LZ1AQ design, and you may be able to get away with it.

    Regards,

    Martin - G8JNJ
  • G8JNJG8JNJ
    edited March 2018
    Hi Marco,

    I have a selection of active antennas running on my KiWi, including some LZ1AQ loops and other whips etc. Which can be chosen using the antenna switching extension. 


    Antenna 5 is a Trask designed 1m active whip and antenna 7 is a loop of the same size as the LZ1AQ's in use but with a higher value of amplifier input impedance. Note how the LF performance is worse, but the HF performance is better than the other loops.

    Although Chavdar's notes are first class, he does concentrate on achieving a very low value of loop amplifier input impedance, which may not always be the best choice, especially if you are more interested in the HF bands. At the higher frequencies, the loop inductance tends to dominate and in some cases, having a much higher value of loop amplifier input impedance can be beneficial, especially if you are only using a thin wire loop, which has a high value of self-inductance.

    Using CAT 5/6 balanced cable can help reduce the pickup of unwanted common mode noise current on the antenna feeder. But don't assume that using balanced cable will completely eliminate it, as it's still possible to have common mode current carried on a twisted pair. 

    Shielded twisted pair has to be treated with extra caution, in case the shield current carries unwanted noise between the two ends of the cable, which may be at different ground potentials.

    Some further notes on various active antennas I have tried and built can be found on this web page http://www.g8jnj.net/activeantennas.htm

    Regards,

    Martin - G8JNJ








  • KA7UKA7U
    Martin,
    This is an interesting circuit.

    Nicely made and priced fairly.
    Ron
    KA7U
  • G8JNJG8JNJ
    Hi Ron,

    A couple of weeks ago I received a private email about this circuit, and decided to make a copy, as the person who wrote was having problems with it, and I couldn't figure out how the circuit could perform as described.

    I found that the IMD performance was poor and it had several other issues which couldn't be easily fixed.

    If you want to build or buy an amplifier of this type, my advice would be to take a look at the DXE RPA-1 circuit instead.

    image



    Regards,

    Martin - G8JNJ




    KA7U
  • G8JNJG8JNJ
    Hi Ron,

    If you want to build a FET amplifier that is likely to work properly, take a look at this design which uses ST PD85004 



    The devices are more suited and are individually biased. 

    However, the noise figure is probably a bit too high for RX purposes.

    BEWARE it is capable of producing several watts of output power :-)

    Regards,

    Martin - G8JNJ
  • KA7UKA7U
    edited March 2018
    Martin,
    Thank you for your insight and suggestions. Currently I'm working through the parts for the LZ1AQ amp.  I'm also curious about the 1000nf capacitors on the antenna input feeds. I'm thinking about a ceramic type. Any recommendations for capacitors in the antenna input line?

    I've ordered in a bunch of Vishay 2n2222 transistors and Motorola TO-39/2N5109 transistors for the 2nd stage. I'm planning to use the CAT6 cable to feed this antenna power, receive line, and rotor control. It is a busy time here, so it will be a few weeks before any results here, I'm sure.

    We also ordered in another 5 Wellgood boards from Mr. Smart. Still waiting on the mail to deliver those. K7RHB and I made 3 of them and our club members are generally impressed with them and asking if we would make more.  The material cost is about $140.00 for a Wellgood loop using 1 inch copper tubing for the loop and standard steel outdoor double gang electrical box.  We found a source for LM317 voltage regulators on a nice board for $2.80 plus shipping, which is very low.
    https://www.aliexpress.com/item/export-DIY-suite-LM317-Adjustable-module-Adjustable-regulated-power-supply-board-With-rectifier-ac-dc-input/32664261259.html?ws_ab_test=searchweb0_0,searchweb201602_5_10152_10065_5711320_10151_10344_10068_10130_10324_10342_10547_10325_10343_10546_10340_10548_10341_10545_10084_10083_10618_10307_5711220_10313_10059_10534_100031_10103_10627_10626_10624_10623_10622_10621_10620_10810_10811_5722415,searchweb201603_25,ppcSwitch_2&algo_expid=9a5dd85e-2324-42ec-a9db-115aecbeebd8-31&algo_pvid=9a5dd85e-2324-42ec-a9db-115aecbeebd8&priceBeautifyAB=0

    So fun stuff to play with.
    Ron
    KA7U

  • G8JNJG8JNJ
    Hi Ron,

    I see that MFJ has rather kindly provided the circuit diagram for their broadband active receive loop the MFJ-1886.



    If you have some money to spare, you could try some of these new Mini-Circuits devices in place of the GAL-74's
     


    Regards,

    Martin - G8JNJ
  • KA7UKA7U
    Martin,
    More things to work with. I've discovered http://easyeda.com . Such a deal and easy to use. Now if we could just get them to assemble the boards too. Hi Hi 

    I've attached a prototype image of the ZL1AQ circuit and the modified schematic. Please comment what you think might make a better circuit for the test purpose.

    Thanks to easyeda.com circuit experimenting is becoming more fun, if not as impulsive. I am having fun with this.
    Ron 
    KA7U


    Attachments:
    https://forum.kiwisdr.com/uploads/Uploader/8b/2644b4eb86f92c30fa9a90dce9705d.png
    https://forum.kiwisdr.com/uploads/Uploader/da/a55d0ac1064e37050830da9aeb019d.png
    https://forum.kiwisdr.com/uploads/Uploader/81/0a7e2c11de9702a92cc29c8d402df7.png
    https://forum.kiwisdr.com/uploads/Uploader/f6/4ac643f711f2b4b1bebd54cee41051.png
  • G8JNJG8JNJ
    Hi Ron,

    t look good to me.

    My only suggestion would be to space out the input filter network and protection diodes a bit more so that you have some space to try different input arrangements.

    For example, if you wanted to add an input transformer, add further input protection, or try different filter components.

    Otherwise, it looks good to go :-)

    I guess you are also building a bias tee / rx interface board to go with it ?

    Regards,

    Martin 


  • KA7UKA7U
    Matin,
    It is setup to use CAT5e or CAT6 feed cable, so I'll use a voltage regulator for the power feed pair, and a 100/50 ohm balun to the receiver in the shack. ZL1AQ shows a balun on the power feed pair. I'm not sure why he does that. Maybe you know? The remaining 4 wires might be have enough current capacity to run a small rotator, but that is to be determined once I try it. Hi Hi. 

    Good ideas on the input real estate. A few extra plated holes might come in handy. I noticed on the MFJ board that the input diodes don't center up on the schematic but rather offset. That seems like a good idea too. What is MFJ doing with the MMD3004BRM diode IC's on that loop amp? That circuit in that location seems curious to me.
    Ron
    KA7U
  • KA7UKA7U
    Martin,
    http://www.mfjenterprises.com/Downloads/index.php?productid=MFJ-1886&amp;filename=1886 3.8 schematic.jpg&amp;company=mfj
    The MFJ loop is perhaps the easiest design of the loop amps. The GALI-74 surface mount amp is just fine in this application. I don't think it could be improved on much, at least as I read the datasheets. I find them @ $17.00 for 5 on eBay and if you order 10, the postage is the same @ $3.00.  The idea of placing the voltage regulator in the case at the antenna is a good one. It prevents voltage drop over a long coax run. There is no need to build either the voltage regulator or the bias-t, both can be purchased at low cost from aliexpress.com:

    and do a search for T-Bias with rectifier at aliexpress.com to find the LM-317 board of choice. 
    So using coax or tubing, and standard electrical box and fittings with an Altoids tin for the Bias-T, this type of antenna should be made for under $50.00 in parts.  Maybe we could convince George Smart to stock a batch of boards for the DIY people. :)

  • G8JNJG8JNJ
    Hi Ron,

    See my earlier post dated March 16th in this thread.

    You may also be interested in this.


    "Anyway, I had a chat with Andy Ikin at Wellbrook, and told him what I was going to do and it just turns out he did a technical evaluation of the MFJ-1886. Here is what he found:
     
    “Mechanically the antenna is very sturdy. Performance wise, it is disappointing when one compares to the established ALA1530. The MW gain is low i.e. 10-15dB below the ALA1530. Overall the HF gain is approx. 5dB lower. The MW IOP2 is approx. +50dBm and IOP3 is +28dBm
     
    The poor IOP2 is mainly due to the absence of a balanced input transformer. The amplifier uses 2 x MIMICs in a pseudo balanced configuration i.e. it uses the output transformer to achieve a balanced input. Hence, by using  50 Ohm input z MIMICs in anti-phase gives an input z of approx. 200 Ohms.
     
    The measured Input z over most of the design bandwidth is quite high at approx. 144 Ohms. Also the input has a 290 Ohm resistor to ground on each Amp. input. Hence, the input z is a combination of the 200 Ohm MIMICs loaded with two 290 Ohm resistors. The amp.  is only matched to the loop in the mid HF band. Normally a 500kHz to 30MHz wideband loop would have a nominal input z of approx. 15 Ohms. Therefore, there is a considerable loop/amp. mis-match at MF plus input attenuation in this frequency range.
     
    Considering that MIMIC amps afford a high noise figure at MF plus an increase in NF due to the low z  loop shunting the amp. I would expect overall NF between 15 to 20dB . This is not going to bode well for NDB listerning.
     
    The Amp. gain at 1MHz is 9dB increasing to a max. of 18dB at 10MHz. I have added 1dB to take into consideration 50 to 144 Ohm mismatch.
     
    The low MW gain appears to compensate for the poor IMD performance.”

    Regards,

    Martin - G8JNJ
  • KA7UKA7U
    edited March 2018
    Martin,
    This is good information. I have been working with the MFJ design:
    MFJ must have changed the schematic since the review, as there is now 560 ohms to ground on the input. https://www.minicircuits.com/pdfs/GALI-74+.pdf
    The data sheet for the GALI-74 (a MIMIC amp) does not support the claim of a high noise figure at MF. That is something that will need to be proven and if true then Mini-Circuits has misrepresented the product. Not that I would know how to test for that. Hi Hi  I've tested the W6LVP Loop, the little unbalanced HiLetgo LNA with shielded magnetic coax loop, and the Wellgood Loop amp. The tests do reveal differences at various frequencies, but in order of test performance from least to best: W6LVP, HiLetgo, Wellgood.  My test method is crude, 5 watts into a 50 ohm load, and place the Loop under test in the same outdoor location, pointed the same way, and read the KiwiSDR dbm result at USB centered on the signal peak. The dbm reading is to the right of the S-Meter on the web browser display. Move the USB cursor to close by section of the waterfall not showing a signal and measure the noise floor. I should add that while there is a difference in performance, each of these antennas are quite good and useable, in my opinion.

    I dropped the voltage regulator and the dual MMBD3004BRM IC's from the MFJ schematic, and placed diode output protection across the output transformer. Voltage regulation and Bias-T is to be at the receiving end of the coax.  They will make 10 of these boards for $2.00 + $20.00 shipping, and the parts list totals $7.80, so less than $30.00 for 10 boards. Amazing prices from this vendor!   I have not ordered any of this yet. I'm still waiting on the ZL1AQ boards Which are expected March 28th from DHL. I thought I'd build a couple of those first and see how they work.  https://easyeda.com/doxnairobi/zl1aq-loop-amp-ron-morell

    I doubt the MFJ design will equal the Wellbrook or the ZL1AQ designs, but I'm having fun trying them out. The Wellgood loop is quite nice, but I really need to clean up my interference sources for these loops to be truly good. My old 210 foot top G5RV dipole fed with 360 feet of buried in conduit RG213 coax seriously outperforms the loops regarding interference over the coax. 

    If you take a look at the circuit board designs, let me know what you would change, and why? :)  I am learning.
    Ron
    KA7U
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