RF Chokes – Fail!

Yes my lack of posting is a result of several fails that built up on making my RF chokes. The chokes themselves were fine but the final build-out and housing did not quite go as planned.  The RG-400 coax was fairly unforgiving in making connections to the chassis mounted SO-259 connectors. I redesigned my enclosure and still had some major issues. As a result, I have decided to build the choke as before with PL-239’s on each end and not enclose them. So off to Ebay to order some suitable connectors. Sigh…

Well I have the connectors now and will re-start my efforts. Should have some additional data and results soon.


Latest on RF Chokes

A 10 foot length of RG400 came in today as scheduled. I wound 12 turns of coax on a FT240-31 toroid. I left about 2 or 3 inches and wound another 12 turns on a second core. This essentially gives me two cores in series:


Here is the choking resistance plot:


I suspect I have some parastic capacitance in this early measurement but I suspect that I clearly have 8K Ohms or more across the range. Here is the loss plot confirming low loss:


And finally the SWR measurement with a 50Ohm Load:


This confirms the fairly low SWR due to using 50 Ohm coax for the windings. Now just need to package this up and install at the rig.


RF Choke with Bifilar Windings – Done!

Here is an RF Choke built using 11 bifilar turns of #14 house wire on a Fair-rite FT240-31 toroid. The choke is mounted in a 3D printed case I pulled off of Thingiverse. K9YC does not recommend mounting an RF choke in an enclosure due to thermal effects but I’ll be running at a max of 100W so I am not expecting this problem.


Here are the measurements made with the miniVNA Pro as described previously. First the choking resistance:


Resistance is greater than Reactance on 80m thru 12m with a choking resistance above 3k Ohms. Here is the Transmission loss which is very close to K9YC’s measurements of similar chokes:


Finally here are the reflection measurements with a 50 Ohm load:


SWR rises to close to 2.5:1 by the time the frequency reaches 30Mhz. This is the only measurment I am a bit concerned with as I want a choke I can place at the rig input. I’ll use two series chokes in this location using 50 Ohm RG400 for the windings. I can use this choke at antenna feedpoints for portable use and any mismatch can be dealt with by the antenna tuner. Cable arrives tomorrow so I should have some comparison data soon.

RF Chokes: Coax vs Bifilar Windings

The type 31 toroids arrived late last week and I started winding some toroids using bifilar windings of #14 THHN House wire. Using the techniques for measurement I confirmed choking resistance to max out in the 4k Ohm range with the range above 2Mhz well above 2K Ohms. The loss was as predicted maxing out at about 1.2dB at 30 MHZ.


When measuring reflectance, the SWR started at 1.08:1 but increases to over 3:1 by the time it gets to 30Mhz. This should not be a problem when used at the antenna feedpoint but I don’t think this is good to hang off the transceiver antenna connector. I am going to use RG-400 coax for the windings for this choke.  This will keep the rig happy. Downside is I have to wait for the coax toi arrive on Friday.

Progress Measuring RF Chokes

I found that I had an unused FT240-43 ferrite toroid so I decided to try and wind an RF choke with it. Remember that an RF choke is the same thing as a 1:1 current balun. I built this one with bifilar windings made from #14 THHN house wiring. The windings are formed by bringing two identical length wires in parallel and tying them together every 3 to 4 inches with a bit of electrical tape. This basically forms a length of transmission line. I wrapped this line through the toroid 11 times and secured them with cable ties. Here are the connections to the miniVNA:

The first image shows what I call the flat orientation that adds parasitic capacitance. The second image shows the connections to the center pin of the miniVNA. I run the calibration right to the end of the gold pins while holding the unit upright. The next mage shows the results when running a transmission sweep with the toroid on the table as shown.

Note the “hump” in the Rs measurement caused by parasitic capacitance. Here is the sweep when help upright away from any surfaces.

Note here the smoother response. I downloaded the data from these readings in Excel format and used the Transmission loss and phase to calculate the Rs using the formulas derived by G3TXQ and found they matched with the readings from the miniVNA. What I need to work on is a 3D printer stand to hold the unit upright and perhaps a fixture to hold the DUT away from any objects that can influence the measurements.

This response is exactly what I was hoping for in that the choking resistance is in the 3.5 to 4kOhm between 40m and 15m. Much better than the 9 turn coax choke. There is strong promise that the type -31 toroid will further improve the low frequency response. I’ll be designing a custom enclosure to 3D print to house the toroid and allow me to add SO-239 chassis connectors.

Measuring RF Chokes with the miniVNA Pro

My bulk order of type 31 toroids is to arrive on Friday. Meanwhile I have been testing some of the chokes I have around the shack with the miniVNA Pro. This turns out to be  more than a bit tricky as parasitic capacitance can affect the readings. Keeping the test leads as short as possible and making sure the choke under test is away from any surfaces such as my desktop. This has the most effect on toroids wound with bifilar windings. The coax wound toroid still exhibits a bit of a resonant hump. Here are a few sample reports after trying to mitigate the capacitance issues:




Ideally I would use a coax with less capacitance to wind toroids with. I plan to use #12 Romex wires from Home Depot to wind bifilar turns and mount in a custom 3D printed enclosure.

I am going to do some additional tests by checking known inductor/capacitor combinations and continue working to reduce the parasitic capacitance.

SDRPlay Spectrum Analyzer Software

I have been playing with some software being developed for the SDRPlay to perform RF spectrum analysis. The software is being developed and can be downloaded here. This should be handy for tracing noise sources as well as evaluating the effectiveness of RF chokes. This can also be used with a cheap noise source to measure filters and with a directional coupler can be used as an antenna analyzer. I’ll be checking this against a similar software for use with the RTL-SDR. here is a screen shot centered around the 1700 kHz AM station in our area:



RF Chokes and Noise

I have previously reported on the positive effects adding RF Chokes to my feed line have had on receive noise. I have come across some detailed information and cookbook on building effective RF Chokes by KY9C. He has recently updated his cookbook for 160-10m which I highly recommend.

I have taken his advise and ordered Fair-rite Type #31 toroids in bulk from Newark to build custom RF Chokes. I plan to build two toroids having 11 bifilar windings from #12 house wiring and put them in series. This should give 8k Ohms choking resistance at 40-20m, 9k Ohms at 15m and 8k Ohms at 10m. I’ll place one right at the rig and another just outside the S9V31 radial field. I’ll be making some detailed measurements with the miniVNA and comparing to some of my existing RF Chokes.

Stress Testing the 12m Spiderpole

I had the Spiderpole fully deployed and holding up a 20m inverted-V dipole for the past couple of days. Winds were quite calm and it allowed me to enjoy the performance of a dipole on 20m. Here is a chart showing current wind conditions:


These winds were strong enough to pull the aluminum channel out of the ground bringing the mast to drop on top of a chain link fence. The mast itself is in good shape but clearly no more 20m Inverted-V dipole for today.

Now looking at how the S9V31+ is holding up. It has been through strong winds before with no issues but today is a bit special. So far it is holding up well but does lean over in gusts. Will keep an eye on this.

Update 1/23/19: crazy strong wind continued all day and sometime during the night shifted from South to North with the front passage. The S9v31+ held up fine with sustained gusts in the 35-45 mph range. Inspection of the Spiderpole installation showed the angle aluminum shifted in ground and there is some deformation at the base. I was unable to pull it fully out of the ground but there is some deformation. Will have to pull it out, straighten it and try and drive it in another few inches. If it fails again I’ll likely need a longer piece.

2019 NA QSO Party

After a lively Radio Club meeting yesterday morning I went home and decided to prepare to operate during the NA QSO party. I got a late start as I wanted to setup for 20m using the Inverted-V dipole up 30 feet on the Spiderpole. It was quite windy yesterday afternoon with gusts up to 30mph from the North. This caused two issues. The first is that the antenna wires would get blown into a nearby tree while extending the Spiderpole. The second issue is that the wind would cause sections to collapse forming the tangled mess I described in a previous post. The first time this happened I noticed the bandscope on the IC-7300 go blank went  outside and saw the mess. After getting it redeployed it collapsed again as I was walking away from it. Once again redeployed the mast and this time it stayed up for the remainder of the time I operated. It collapsed again at some point just after sundown when the band had closed.

All in all the 20m Inverted-V performed well. Once I started using it I would get “big signal” comments on occasion.  20m was the band to be on during the afternoon hours although 15m seemed to be open to the west cost.75% of my QSO’s were on 20m. The 20m bandscope as very lively:


Overall score was 98 QSO’s on three bands for a score of 4508. Lot’s of fun for a Saturday afternoon of radio!