NanoVNA - S11 and MFJ-264 dummy load - Part 2 (performance improvements)

A follow-on to a previous video looking at S11 measurements of an MFJ264 dummy load. This is another film in an evolving set of educational videos featuring the NanoVNA. This one overviews transmission line theory and how it is used in UHF loads, and illustrates how the NanoVNA can be employed to understand and improve high frequency electronic designs.

[@craxd1]

This was why Collins used a 37 pF doorknob capacitor in their DL-1 dummy load, which is in parallel with the resistor bank to ground. I think a few others used a cap as well. One company used an air variable, if I recall. I remember speaking of this, about the Palstar loads, with their company years ago.

[@MegawattKS]

Interesting. I pulled the manual for the DL-1 to get a look. My guess is that the cap performs a similar function to the internal shield in the MFJ (working with the series L of the resistor bank) - but without the benefit of being broadband. With the relay in series with the signal line, there an additional series inductance as well. With the cap added (to the existing C from the resistors to case), they get one LC section of a discrete LC transmission-line model. Probably improves the high-frequency SWR performance some. But alas, I suspect the DL-1 doesn't work well above 30'ish MHz. I can't actually find a spec on it, or any SWR (or RL) measurements versus frequency. Any idea how high it goes? collinsradio.org/archives/manuals/DL1%20Dummy%20Load.pdf

[@vironpayne3405]

Engineering majic with the right knowldge and right electron glasses. Thank you!

[@macspillers4731]

Genius... The outro of still images and music is such a little thing but helps so much in teaching. It is quick but recaps the essence without explaining too much, so the viewer has to think to remember the key points. Excellent content!

[@MegawattKS]

Wow - thanks for noticing/commenting ! Sadly I didn't do this on lots of the videos I've done since, but a few do have it.

[@MrBanzoid]

Nice investigation. I just fell over you channel an duly subbed. Thanks.

[@maksberlec4832]

Excellent video!

[@MegawattKS]

Glad you liked it!

[@bjrapp]

Very interesting!

[@stevesilsby5288]

Very well done. I'm looking forward to more! de WA4BRL

[@MegawattKS]

Thanks. I'm thinking of doing one next on antenna design. Might be a while though. I seem to crank these out slowly ;-)

[@JacquesConradie]

Quite good - thanks!

[@MegawattKS]

Glad you liked it :-) Thanks for the feedback.

[@RickGreenPhoto]

What if you put the resistor in a metal tube that is grounded to the front connector directly and is of an appropriate diameter also grounded to the opposite side of the resistor. So basically you have a large resistive center conductor with a 50 ohm impedance and resistance..

[@MegawattKS]

Sounds good I think. The existing design is probably a compromise between performance and cost and other drivers - but what you said sounds like the ideal. Keep it coaxial the whole way :-) (with the proper diameters of inner and outer conductors)

[@user-xj8oh6fo2f]

Great demonstration, thanks! Interesting what does MFJ think about it? ))

[@germanjohn5626]

Mighty Fine Junk doesn't care because it sold for ham radio and they don't go up there they might test at 450Mhz not 600. Buying anything from MFJ is a compromise in performance and quality on par with buying Chinese junk on e-Bay. That said, that load is quite lousy even on 2m. I build dummy loads for my own use and none of them has a return loss worse than 30db a number I deem satisfactory but not perfect. I like to see how this load does with a power level of 1.5Kw on 144Mhz or 432 and any prolonged time lol I think someone might have to call the fire department trying to do that. Those standoffs will glow white hot lol.

[@davesanders4381]

Have you considered adding a very short copper grounding strap from the flange on the SO-239 connector to the cage?

[@MegawattKS]

Sounds like a potentially good option - but I'd want to support it somehow to keep the distance from the center-conductor/bracket to prevent RF arcing. I calculate 274V (350Vpk) at 1500 Watts (if it is operating at 50 Ohms). But not sure what that implies about gap requirements. (Fortunately I don't have use for this thing above 440 MHz, so it's working OK as is. I just found it to be a fun puzzle to solve with transmission line theory :-)

[@maxastuto]

HI , i am an italian Ham IU8MLV I have an MFJ 260C, It would be possible to lower the swr from swr 1: 1,44 (in UHF 430-436 Mhz) to 1: 1,04. that is, from a return loss of -14.5 dB to -32.5 dB, From 1 to 30 Mhz it has a SWR of 1: 1.04, in VHF (144-146 Mhz) it has SWR 1: 1.2. Is it possible to add in parallel on the RF connector of the dummy load some high voltage ceramic capacitors from very few pF to make the SWR in UHF lower? Many Thanks 73 51

[@MegawattKS]

Not sure. I'm not familiar with the 260C. But for many situations a return loss of 14 dB is not too bad. I'm often happy with RL of > 10 dB, although 20 dB is often my goal. Anything beyond that may be influenced by the measurement equipment and setup. If you need the -32 dB S11, maybe try a matching network? Of course that will only work for one frequency. For braodband, you'd have to play with the dimensions of the structure and look for parasitic inductances/etc like the 264 seemed to suffer from. Sorry I don't have better info on how to improve a 260C. 73's

[@therfnoob7697]

nice! Subscribed! It would seem a rather poor design from MFJ! Anyway, I am not sure I understand the relationship with the original problem and the 12.7cm... I understand it was an educated initial clue/guess but, ultimately, do you think it was really the length of the "cage" to cause (or have some part in) the problem? It looks like your fix solved the problem, while the 12cm 'cage' is still there. Thanks for any comment!

[@MegawattKS]

Thanks ! Yes - the length of the cage was just a clue along the way as you surmised. 1/4 (or 1/2, or 3/4, ...) wavelength is a common regime in which to find interesting behavior in transmission line effects (which everything involves at high enough frequency due to dimensions and speed of light). But as you pointed out, the design clearly works once more grounding of it is done, and even without additional grounding it works great at lower frequencies. S11 stayed good up through the 70cm ham band - so not a problem for 99% of likely customers and applications. FWIW - MFJ and Alinco have always been a couple of my favorite companies. Generally solid products at good pricing and with good features and user-interfaces IMO. :-) Thanks for watching and commenting ! 73's

[@warplanner8852]

Paging MegawattKS, Martin F. Jue's attorney on line two!

[@jackpitw]

The latest generation of the MFJ-264 seems to have incorporated your "fixes"....

[@MegawattKS]

Interesting ! That's great news. For the record, I have never contacted them - so maybe they did it themselves, or maybe they saw the video. Either way, it's nice 🙂

[@bruceblosser384]

As usual! MFJ stuff is Quick Cheap and Dirty! No MORE MFJ for me!