Wellgood LNA, 1″ copper X 45″ diameter loop, and Bias-T coax power

The Hiletgo LNA and the 1 meter diameter coax loop made such a nice antenna I thought it might be interesting to try another version of an active magnetic loop antenna. So I asked Bob K7RHB to help me to make the Wellgood boards and other parts for the project. We have worked on this together and having lots of fun in the process.

George Smart has parts for one at his website:
https://www.george-smart.co.uk/projects/wellgood_loop/  ,and so this is the LNA we decided to build and try out. Unlike the Shielded Magnetic loop made for the unbalanced Hiletgo LNA, this LNA is a balanced amplifier and therefore the loop does not need a center conductor to provide the virtual balun effect to feed the LNA. Just a copper loop is all that is needed to feed this balanced amplifier.  We used soft drawn 1″ copper tubing and it is easy enough to form into a loop as it is unrolled from the box it ships in.  The CPVC tee is not really strong enough to support the loop in bad weather, so a center support to the top of the loop is a good idea.

After the Wellgood LNA was assembled it seemed to work best with a voltage of 3.1 VDC.  The first boards used the  2N3866 transistors and these did not exhibit the gain over voltage increase that we thought they should, so we changed them out with 2N5109 transistors. These transistors seem to operate with better noise floor and gain linearity over a voltage range. We selected 5vdc as the operating voltage for these transistors. A Bias-T printed circuit board is provided with the LNA boards so we soldered up the parts for the Bias-T and made a LM317 voltage regulator from the circuit supplied with the data sheet for that device.

The Bias-T and LM317 board fit nicely in a small cast aluminum box and a 14vdc linear transformer wallwart provides the power to it.

After making the LM317 regulator I learned that http://aliexpress.com practically gives away the LM317 regulators built out for very little cost. You just have to have a month or two to wait for it to arrive.

So does it work better than the Hiletgo Shielded Magnetic Coaxial Loop?  Well yes it does, but I’m not sure if that is due to the Wellgood LNA or the larger diameter loop.  In order to have a good comparison I need to make a 1″ coax into a 45″ diameter loop and then take comparison readings. I suspect the loop balance with the Wellgood design will provide equal gain end to end and good nulls side to side, while the Shielded Magnetic Loop may not exhibit that excellent balance. Still, objective testing has not yet been done here to really know.

Having fun with the active loop antennas!
73,

Ron
KA7U

Home Brew Active Shielded Magnetic Coax Loop

An affordable do it yourself active loop antenna that covers 15KHz to 32MHz and probably more, is roughly represented by the following sketch.

NOTE: at the top of the antenna in this sketch the center conductor is connected to the shield. The antenna will work fine by not cutting the center conductor and not connecting it to the braid at the top of the antenna. Just connect it at the bottom of the loop.

To further describe this antenna shown in the sketch, it is 1 meter in diameter more or less, and the coax could be any coax that is available. I used a foam filled RG8 which is very flexible and doesn’t hold a shape. LM400 or other solid dielectric coax should be a better choice. I was experimenting on this build and I’ll choose better material on my next build.The Hiletgo LNA is an unbalanced amplifier that works from 100KHz to 2GHz according to the spec. I find it works from 15KHz to probably 2GHz…, but in my case to 32MHz for sure. At the time of this writing this amp is available at Amazon.com, search for:
HiLetgo 0.1-2000MHz RF Wide Band Amplifier 30dB High Gain Low Noise LNA Amplifier
I paid $10.99 for it delivered with Amazon Prime.

This Hiletgo LNA works with power up to 12vdc but I found that it only takes the 8 vdc to work down to 15KHz effectively. The DC power needs to be clean DC and the voltage should not exceed 12VDC or you may experience noise issues with the little amp. I used power over the coax to power it and so I needed to add a 1mh inductor from the coax center conductor to the +vdc input on the board. This being placed on the line side of the little input capacitor already inline on the board effectively isolates the input power from the LNA input. See the following image.

Aliexpress.com is currently selling a low cost Bias-T that works down to 1MHz. I suspect it will be fine to use with the Hiletgo LNA. Search for Tee Bias on the website.  There is also a 9vdc clip soldered to the power input, but that was just a temporary testing solution. You might notice the Hiletgo LNA has SMA connectors in and out and I chose to use those to make connections to it with coax pigtails and adapters for testing. These could be removed if the board was to be installed permanently in a box and then connections soldered. I will probably continue to use adapters for ease of changing the amplifier even if I put it in a box or pipe.

The following image shows the current connections to the antenna and LNA.

 

 

This antenna is currently connected to a KiwiSDR at http://ka7u.no-ip.org:8073 . It won’t stay there long as a new one will likely be installed soon.

 

And sure enough, here is the updated version of the loop antenna. The 1/2″ PVC pipe is not glued and is depending on compression and friction to stay together. This makes it easy to take apart and use portable.

So here is an image of the waterfall shown side by side with a waterfall image of a 210′ dipole for comparison.

 

It occurred to me to check the relative antenna / LNA. 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.

28.5MHz    –   -69dBm
24.9MHz    –   -63dBm
21.2MHz    –   -48dBm
18.1MHz    –   -44dBm
14.2MHz    –   -54dBm
10.12MHz  –   -53dBm
7.15MHz    –   -57dBm
5.33MHz    –   -59dBm
3.75MHz    –   -67dBm
1.9MHz      –   -90dBm
The 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.

Inspiration for experimenting and building this loop is taken from the following web sites:

https://pa0fri.home.xs4all.nl/Ant/Active%20antenna/Active%20receiving%20%20loop%20antenna%20eng.htm
https://w8ji.com/magnetic_receiving_loops.htm
http://valentfx.com/vanilla/categories/kiwisdr-discussion
https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm

Video discussing setting the Hiletgo LNA for best SNR. If you don’t want to watch the video, use 8 to 9vdc and you should be fine. 8vdc seems to give the lowest noise floor and 8.5vdc might be just a bit better on SNR.

 

 

Ron
KA7U