Best MWDX antenna?

[~SIGINT~]

Snap a close-up picture some day once you have a completed version. I would like to see your build.

[ThaDood]

Well, this looks more convenient than that godzeyes antenna that I've made.  https://swling.com/blog/2024/01/video-giuseppe-explains-wiring-construction-of-dica-homebrew-antenna/   Tis the season to build and try stuff.

[RobRich]

I will try to post pics if/when assembling another one. I did not even bother with a board or enclosure this time. Instead I just used heat-shrink tubing to hold the F connectors on each end, and that tends suffice for an indoor transformer that is not going to moved around a bunch.


I left a comment on his other ferrite antenna blog post:

https://swling.com/blog/2024/01/d-i-c-a-giuseppes-latest-homebrew-portable-antenna/#comments

Seems you can get away with a rather small type 61 ferrite even well above the tropical bands if opting for preamp stage. This design suggests possibly up to 12MHz:

http://www.jumaradio.com/active-ferrite-rod-antenna/active-ferrite-rod-antenna.html

[~SIGINT~]

No worries on the pictures. I was just curious what you had used to construct the transformer since you were mentioning magnets.

You did get me going out looking at antenna galvanic isolators again. I have always been interested in the Bonito but I find that it is expensive.

I did come across a couple of interesting links. Heros Technology has a unit which you can transmit into (200 watts) and has low insertion loss. They also have some good pictures and PDFs on the site.

The other is from RF SYSTEM. Download the PDF from the link below. This is very close to what you are building. They use a transformer from MINICIRCUITS. I find that the insertion loss on this transformer is high (1 dB). Maybe MINICIRCUITS has a better transformer than the one selected for this product.

Heros Technology
https://www.herostechnology.co.uk/pages/RF_Galvanic_Isolators.html

RF SYSTEM
http://www.rfsystem.it/
http://www.rfsystem.it/pmsdr/files/Antenna_isolator.pdf

[RobRich]

Fair-Rite BN73-302 (2873000202) mix 73 binocular core. Currently ~$0.68 each at DigiKey and Mouser. Four turns on the antenna side. Four turns on the receiver side. I have seen some mentions of trying 5:5 as well, though 4:4 usually seems to work fine for my purposes.

My noise usually tends to be lower-HF downwards, so I generally favor the BN73-302. Fair-Rite has an affordable mix 43 binoc (2843000102) as well. Not sure I have ever seen a mix 31 binoc.

A possible alternative to the BN73-302 might be a TDK N30 binoc core. I keep planning to try N30 ferrites, but I have yet to order any.

https://dh1tw.de/2021/06/whats-the-best-ferrite-material-for-a-common-mode-choke/

I am not too concerned about insertion loss, especially for receiving under 10MHz, as even my loop-on-ground has plenty of signal for its intended operating frequency range.

Curious, have you skimmed any of Coilcraft's offerings? It has SMT transformers usable at MW/HF frequencies rated down to 0.2dB insertion loss assuming that is a concern. There are some through-hole models as well, but I did not see printed loss numbers in a quick skim.

https://www.coilcraft.com/en-us/products/transformers/wideband-rf-transformers/#/

[~SIGINT~]

Very nice on the Coilcraft offerings. I was not aware of them. It looks like a WB1010-PCL, WB2010-PCL or WB3010-PCL (0.005 to 100 MHz) would do the job for HF. Thanks.

Here is a link with a comparison of response vs. number of turns. I wish I could make heads or tail of those frequency charts. What a horrible way to express the frequency.

Measurements of some transformers for low-bands antennas
https://www.qsl.net/in3otd/ham_radio/160m_transformers/160m_trafos.html

[RobRich]

Took a quick skim. I have a degree in math, and yeah, even I think simply stating for example 10MHz is much more readable than 1e7 (1x10^7) Hz.

The "braid breaker" design is showing measured losses for 2:2 isolated straight windings on mix 73 should be okay for LW to mid-VHF, though I noted he is using two stacked binoc ferrites. Correlating against his bifilar charts, it would seem 3:3, 4:4, and 5:5 on mix 73 should have little real-world differences for MW/HF losses.

Looking at the capacitance chart, with ranges of 0-5pF and 1-100MHz, I would think the measured numbers are low enough at those frequencies to be largely trivial unless doing a tuned circuit.


With the mention of a tuned circuit, the idea of doing a remotely tuned HF receiving loop using a varactor circuit is somewhat interesting. For example:

http://techlib.com/electronics/antennas.html

[~SIGINT~]

The circuit fits nicely in the standard PVC junction box although I would question the use of braid as an antenna conductor for the same reasons that braid is not ideal for grounding.

[RobRich]

Lots of loop antenna designs use coaxial braid, but I agree, especially since a 10' roll of 3/4" soft copper refrigeration tubing is like $10 to $15 at Lowes and similar. That is about the same cost as 10' of bulk RG-8/213.

Aluminum should suffice as an alternative as well, especially considering a preamp is used in the circuit. Here is a rather simple varactor-tuned loop based upon the previously linked design, but using Al tubing:

https://sivantoledotech.wordpress.com/2010/09/18/a-tuned-active-receiving-loop/

Noted some RNoted some RFI tonight. Brought my hulahoop loop on a tripod inside. Using upper MW as a test band, I could null much of the RFI, so it is likely somewhere in the area.


Just got back to the radio, and the RFI is gone now at ~0750z.

Anyway, in the meantime, I did manage to toss together a mix 73 isolation transformer on piece of perf board to replace the temporary one I built a few days ago.FI tonight. Brought my hulahoop loop on a tripod inside. Using upper MW as a test band, I could null much of the RFI, so it is likely somewhere in the area.


Just got back to the radio, and the RFI is gone now at ~0750z.

Anyway, in the meantime, I did manage to toss together a mix 73 isolation transformer on piece of perf board to replace the temporary one I built a few days ago.


31' vertical without mix 73 isolation transformer near receiver. Notebook on power supply.


(click to enlarge)

31' vertical with mix 73 isolation transformer near receiver. Notebook on power supply.


(click to enlarge)

Not that I actually recommend using a 31' vertical for VHF reception, but I did take a quick look at area NOAA broadcasts with and without the transformer. SDR floor and peak numbers suggest around 2dB or so of transformer loss at mid-VHF frequencies. Factored against typical mix 73 characteristics, that would suggest LW/MW/HF losses are likely quite negligible.


Thought I might have to look at the feedline and the hybrid feedpoint unun on the 31' vertical, as my 148' loop-on-ground even with its incurred losses was showing considerably more gain on many bands. o.0 Tightened various connections on my inside feedlines. Thankfully it seems back to normal.

I do need to replace an antenna switch. I have one of those inexpensive CATV slide switches in line with my Airspy HF+D.

I still have that WiFi remote antenna switch collecting dust on a shelf. It passes DC from the bias tee to the antenna ports, so I would it would need DC blocking caps. Suppose I could add the caps internally, but taking the hopefully easier route, I just ordered some CATV DC blocks to try first.

(^Merged posts.)

[RobRich]

I suspect the screenshot below is about as good as the 31' vertical is going to get at VLF during the daytime with my usual notebook running on a switching power supply. The noise floor is approaching -130dB on 60KHz,

I added a -3dB attenuator (for now?) between the coax switch and mix 73 isolation transformer, of which the other side connects to my Airspy HF+D via a RG-316 jumper wound on a (mix 43?) snap-on ferrite. I probably could get away with more like a -10dB or even a -20dB attenuator (on some bands) to perhaps slightly help with some of the mismatches involved.


(click to enlarge)

On a related note, I added (IIRC) a -10dB attenuator to the active miniwhip feeding my nightstand receiver as well. I need to check it again, but in a quick skim, it seems to have helped to further clean up the cheap MSI SDR being used there.

[~SIGINT~]

Nice clean-up. Is it the power supply or the notebook generating the noise? The reason I ask is because my IBM ThinkPad is similar. Although the switching power supply is noisy when I plug it into the laptop the entire laptop becomes an enormous source of noise, more so than the power supply.

[RobRich]

It is mostly the power supply. I have two HP 150w notebook PSUs. The -130dB screenshot is a somewhat older model after deploying the various antenna, feedline, etc RFI mitigations..

Last time I checked I seem to remember the newer model pretty much being a RF generator. o.0 I keep intending to modify the noisy one, but it apparently is not high on the priority list.

I was considering migrating to a 19v linear PSU, but it seems largely mitigated for now, at least for my 31' vertical. Perhaps later.

On a higher priority note, I need to get around to sourcing a decent 5v PSU for the KiwiSDR2 on order.

[~SIGINT~]

Plenty of reasonable choices on eBay for the power supply.

Lambda LNS-X-5-OV power supply 5 vdc 10 amps out.
https://www.ebay.com/itm/293812347657

Lambda LDS-P-5-OV Massive Linear Power Supply +5v DC 22A
https://www.ebay.com/itm/275385083025

Lambda LP-520-FM Regulated Power Supply 0-10VDC 3.7-5.0A
https://www.ebay.com/itm/266607613314

Hewlett Packard 62005E DC Power Supply 8 Amp 5vdc
https://www.ebay.com/itm/304807918132

HP6281B Bench Variable Power Supply 0-7.5V 0-5A
https://www.ebay.com/itm/134002920421

[RobRich]

A quick skim of specs for the Lambda LNS-X-5-OV looked pretty good IMO. Ordered.

I redid some inside feedlines and removed the -3dB attenuator on the Airsphy HF+D feedline. LW/MW on the 31' vertical still seems okay. WWVB and sub comms present. Medi 1 on 173Khz is there but largely buried in static crashing tonight.

[~SIGINT~]

Excellent on the power supply. One thing with the Lambda power supplies is that most are ment to be mounted as part of a piece of equpiment so they do not have any on/off switches and are un-fused. Make you you add your own in-line fuses for protection.