Sunday, June 28, 2015

Antenna Auto-tuner Design, Part 4: Relays and L-Network Schematic (Preliminary)

[Update, 21 June 2016:  I received a comment from Jukka Siitari, OH2AXE correcting some questions (and misconceptions I had regarding the relay specifications.  You can find the full text of his comment in the 'comments' section following this post.  I have also inserted his comments into the body of the blog post text.]

In this fourth post I thought I'd look into relays. (Part 3 of this series can be found here).

I noticed that Elecraft uses the Omron G2RL series in their KAT-500.  The "high capacity" version of this family (which they use) is spec'd at 16 amps.

Looking at the Digikey website for other high-current low-profile relays, I found one made by Panasonic (their ALZ family).  Coincidentally, it has the same specs as the Omron relay.

So that I could compare them, I ordered a few of the Omron and the Panasonic relays from Digikey.  Here are some photos...

Omron G2RL relays on the left, Panasonic ALZ relays on the right, all with covers removed.


Omron G2RL-14-E Relay, view of contact mechanism.  Note the width of the copper:


Omron  Relay, normally-open contacts forced closed (12VDC applied):


Looks pretty good!

Panasonic ALZ12B12 Relay, normally-open contacts forced closed:


Hmmmm.....It looks like the contacts aren't contacting center-to-center, but rather at their top. Personally, I prefer the Omron relay -- it looks like it the relay contacts connect at their centers.

Looking more closely at the Omron Relay specs...

The important Omron Relay Specs:
  • Contact Material:  Ag-alloy (Cd Free).  (Note that an earlier datasheet states AgSnO2 for single-pole relays; AgNi for double-pole relays).
  • Contact Resistance:  100 milliohms, max.
  • Operating Current:  16A
  • Max Switching Voltage:  440VAC, 300VDC. (I'm confused as to why the AC voltage is higher than the DC voltage -- I would think that the DC voltage would be Vpk of the AC, or about 600 volts.)  [Comment from OH2AXE (20 June 16):  they are like this because DC arcs easier than AC when contact opens with full load (ref. welding). These are power relays intended for DC or mains frequency. RF is even worse, because it ionizes air and causes an arc easier than 50/60 Hz DC. You have noted this later in the text. ALWAYS tune with a much lower RF power and arcing should not be a problem.]
  • Dielectric Strength Between Contacts "of the same polarity":  1000 VAC, 50/60Hz, 1 Minute.  (I'm confused by what the meaning could be of "contacts of the same polarity", but I believe this spec is the same as Panasonic's "Breakdown voltage between open contacts.")   [Comment from OH2AXE (20 June 16):  This specification is valid only for relays with 2 or more "poles", in this case DPDT version. It is the breakdown voltage between two adjacent contact sets. Your assumption leads to significant errors later in your text. For HF frequencies use the DC switching voltage (300V in this case) + safety margin. You note this later ("Recommendations for Transmitter Site Preparation"). ALWAYS tune with a much lower RF power and contact breakdown (ie. arcing over) should not be a problem.]  (Thanks for the clarification!  - Jeff, K6JCA).
Let's check that this breakdown voltage is OK for my tuner application...

At low frequencies (e.g. 50/60 Hz), the breakdown voltage is 440 VAC.  But for HF RF signals the breakdown voltage is lower.

Per the image below (from Nautel's "Recommendations for Transmitter Site Preparation, Sept., 04", available here), for the frequencies that interest me, the breakdown voltage is about 0.8 times the low-frequency AC breakdown.

(click on image to enlarge)

Given this relationship, the RF breakdown voltage would be equal to 0.8 * 440 Vrms (about 350 Vrms, or about 500 Vpk).  [Note that OH2AXE (see comment section, below) recommends that the DC breakdown value of 300 VDC be used as the RF breakdown voltage.]

As shown in the table below of simulated L-network voltages and currents, the peak voltage across the capacitors can exceed the voltage-breakdown value (I've assumed a peak power level of 800 watts):

(click on image to enlarge)

[Note from K6JCA, 21 June 16:  Per OH2AXE's comments, the relays should have a higher spec'd breakdown voltage for this application.  But the tuner has been built and it is too late to modify it. The good news is, I haven't yet had any problems with contact arcing.  If I were to build another tuner, I would seriously consider using DPDT relays with their contacts wired in series (i.e. doubling the breakdown voltage), although this technique would also increase the series resistance and inductance in the RF path (due to relay inductance and contact resistance.  Also, I recommend tuning only at low RF power.]

Relay Measurements:

I made a couple of measurements on a sample-of-one G2RL-14-E relay:

Contact Resistance: 0.005 ohms (measured between the N.O. contact and the pole, with the relay energized (contacts closed), using an HP3468A DVM in 4-wire resistance measurement mode).

Contact Capacitance: 1 pF (approx) (measured between the N.O. contact and the pole, relay off, using a GenRad 1657 Digibridge).

For relay inductance, I calculate that it should be around 12 nH (Assume internal path length of 1” and contact width of 0.34” (0.87cm)).

Omron G2RL-2 DPDT Relay:

I thought I'd also take a look at the construction of the DPDT member of Omron's G2RL family, the G2RL-2:




Unlike the G2RL-14-E SPDT relay, this relay is only rated for an 8 Amp load.

Other notes on the Omron Relay:

"Sealed" seems to refer to the sealing of a tiny hole on the top of the relay package.

I really didn't see any difference between UL Class B and Class F Insulation.  To me they looked the same, but I might have missed something.

With these relays and with the inductors and capacitors selected in the previous, two posts, I'll  draw a preliminary schematic of the L-Network components and connections:


L-Network Schematic:

This schematic only contains the inductors, capacitors, and relay contacts.  The relay magnets and other circuitry will appear in schematics later.

(click on image to enlarge)

In its Power-Off state, the tuner defaults to Bypass Mode.  That is, the capacitors are disconnected and the inductors are all shunted.  There will be some parasitic effects, though, due to the capacitance of the capacitor-relays' open contacts and the inductance of the inductor-relays and the wires (or traces) interconnecting them.  I'll have to see if these are an issue when I build the network.

>>>  Important Note!!!  <<<

Schematics will change as this design progresses.
Be sure to check later postings in this series 
(especially Part 7) for later versions.

OK -- that's it for this blog post!

The previous post in this series, Part 3, can be found here: http://k6jca.blogspot.com/2015/06/antenna-auto-tuner-design-part-3.html

And the final "release" schematics can be found in Part 10 of this series:  http://k6jca.blogspot.com/2016/01/antenna-auto-tuner-design-part-10-final.html


Links to my blog posts in this Auto-tuner series:

Part 1:  Preliminary Specification

Part 2:  Network Capacitor Selection

Part 3:  Network Inductor Selection

Part 4:  Relays and L-Network Schematic (Preliminary)

Part 5:  Directional Coupler Design

Part 6:  Notes on Match Detection

Part 7:  The Build, Phase 1

Part 8:  The Build, Phase 2 (Integration of Match Detection)

Part 9:  The Build, Phase 3 (Incorporating a Microcontroller)

Part 10:  The Final Schematics


Resources:

My Other Tuner posts:

A quick tutorial on Smith Chart basics:  http://k6jca.blogspot.com/2015/03/a-brief-tutorial-on-smith-charts.html

The L-network:  http://k6jca.blogspot.com/2015/03/notes-on-antenna-tuners-l-network-and.html

A correction to the usual L-network design constraints:  http://k6jca.blogspot.com/2015/04/revisiting-l-network-equations-and.html

A look at highpass T-Networks:  http://k6jca.blogspot.com/2015/04/notes-on-antenna-tuners-t-network-part-1.html

More on the W8ZR EZ-Tuner:  http://k6jca.blogspot.com/2015/05/notes-on-antenna-tuners-more-on-w8zr-ez.html  (Note that this tuner is also discussed in the highpass T-Network post).

The Elecraft KAT-500:  http://k6jca.blogspot.com/2015/05/notes-on-antenna-tuners-elecraft-kat500.html
 
 The Nye-Viking MB-V-A and the Rohde Coupler  :  http://k6jca.blogspot.com/2015/05/notes-on-antenna-tuners-nye-viking-mb-v.html


Standard Caveat:

As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations.  If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.

And so I should add -- this design and any associated information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

2 comments:

Jukka Siitari - OH2AXE said...

Thanks for a well written and in-depth treatise about building an automatic antenna tuner!

Just in case you haven't found these out by yourself, I have some comments concerning your notes about the Omron G2RL-14-E Relays:

1. You write: "Max Switching Voltage:  440VAC, 300VDC. (I'm confused as to why the AC voltage is higher than the DC voltage -- I would think that the DC voltage would be Vpk of the AC, or about 600 volts.)"
My comment: Sorry, they are like this because DC arcs easier than AC when contact opens with full load (ref. welding). These are power relays intended for DC or mains frequency. RF is even worse, because it ionizes air and causes an arc easier than 50/60 Hz DC. You have noted this later in the text. ALWAYS tune with a much lower RF power and arcing should not be a problem.

2. You write: "Dielectric Strength Between Contacts "of the same polarity":  1000 VAC, 50/60Hz, 1 Minute. (I'm confused by what the meaning could be of "contacts of the same polarity", but I believe this spec is the same as Panasonic's "Breakdown voltage between open contacts.")"
My comment: Sorry, it is not! This specification is valid only for relays with 2 or more "poles", in this case DPDT version. It is the breakdown voltage between two adjacent contact sets. Your assumption leads to significant errors later in your text. For HF frequencies use the DC switching voltage (300V in this case) + safety margin. You note this later ("Recommendations for Transmitter Site Preparation"). ALWAYS tune with a much lower RF power and contact breakdown (ie. arcing over) should not be a problem.

3. You write (below the "High Frequency Versus Static Breakdown Voltage" chart): "Given this relationship, the RF breakdown voltage would be equal to 0.8 * 1414, or about 1130 Vpk, which would be equivalent to 800 Vrms."
My comment: Sorry, this is a wrong assumption! The correct voltage is 300 Vpeak or 213 Vrms! You assume the contact DC breakdown voltage to be 1414V, while it is specified as 300 VDC.

4. You write: "Sealed" seems to refer to the sealing of a tiny hole on the top of the relay package."
My comment: Yes, but there is more to it. Sealing protects the relay innards during flow-soldering process. The spec sheet says: "The G2RL model is flux-resistant with two sealing holes on the case. Thus, do not clean the Relay by boiling or soaking in water. ..."

Thanks again for a thought-provoking series of articles. I especially appreciate that you have documented all the problems you have had.

I am also designing and building an automatic antenna tuner using the same principles, but mine will be remote controlled, to match at least 10:1 SWR on all ham HF bands (not 6 m) very close to 1:1, and for 1500W continuous TX power. Tuning will always be done with about 50 Watts (or less - firmware limited). One of my major concerns was to find relays to do the job. That's the reason for the relay comments above - HI. I'm now using Omron's modified LY2-24VDC relays (DPDT originally, after modification 1 NO contact and 1 NC contact, no COM terminal).

73 de
Jukka - OH2AXE (also SV9RMU)

Jeff said...

Many thanks for the comment, Jukka. I very much appreciate your clarifications and I have incorporated them into the blog post.

Regarding the relay's RF Breakdown Voltage...per the graph in the post, I would think that this voltage would be 0.8*440 VAC (where 440 VAC is the relay's 50/60 Hz breakdown voltage), rather than 300 VDC. Is there a reason why it would not be this value?

Again, many thanks!

Best regards,

- Jeff, k6jca