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Choosing proper switching devices for a DIY project (auto relay, rf relay, glass reed etc)

Discussion in 'Homebrew and Kit Projects' started by SP4IT, Nov 13, 2021.

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  1. SP4IT

    SP4IT Ham Member QRZ Page

    I'm planning my DIY receive antenna selector switch. It'll have 6 inputs, 4 outputs (no splitters, one to one connections only), automatic grounding of all inputs when not in use as well as when external TX signal is input. Also receiver protection.

    However, I'm wondering what type of switching to use. I initially considered rotary wafer switches. I suppose those could work fine if thin coax was used to make connections. It is still an attractive option, but I'm also considering relays and reeds for potentially better isolation and future remote control possibility.

    HF rated relays are a buy too expensive for this project so I'm looking at alternatives. Mainly standard Pcb mounted miniature relays or auto relays. I've also recently bought some very cheap glass reed tubes. After wrapping one with thin magnet wire I have a tiny relay that I expect will work much better than submitiature dc relays on account of the Reed being more like a straight piece of wire when closed.

    So if you have experience switching receive antennas with wafer switches, non-hf rated submitiature relays or glass reeds, please let me know your thoughts.

    I'll be thinking about a transmit path switch in future, but that will need much higher rated switching devices so I would like to concentrate only on receive side here.
     
  2. PU5VEN

    PU5VEN Ham Member QRZ Page

    For HF most general use relay will work.

    For VHF and UHF you must be careful about the choices. Check the Axicom IM series (IM06GR for instance). Although they are signal relays they have fairly good RF Data specs (37dB isolation, 0,03dB insertion and 1.06:1 SWR @ 100Mhz) and are very cheap.

    If you decide to solve the TX path as well there are Dow-Key and RelComm options on eBay for $25 or less, 500W on VHF, 80dB+ isolation
     
  3. KK4NSF

    KK4NSF Ham Member QRZ Page

    I use a 6input / 2 output switching box for my receive-only system, and had to build several boxes before I got what I wanted. For Receive-Only I found wafer switches to be the most reliable. However, since I could not find a wafer that grounded the unused inputs, I had to make my own out of PC board. It wasn't very hard to do using ferric chloride and a small jewlers saw..... but it's not a simple "assemble parts" type of build.

    Basically, it has two switches- the input selector is a 6 pole / 2 throw. One goes to the output selector, the other 5 go to ground. The output selector is a simple 2p/2t switch. All of the connections are thin coax with the braids grounded.

    That system is excellent for the receive only usage it was designed for. However, I would not tust it on transmit. The problem is that I'm not sure it would not send a little RF from radio A to radio B.
     
  4. SP4IT

    SP4IT Ham Member QRZ Page

    Thanks. Do you think grounding unused antennas is really necessary? I was planning to short them to ground via a choke or a large resistor permanently to prevent static electricity build up. I'm planning to use 90V GDTs for lighting(and serious overvoltage) protection.

    Did you need to ground unused inputs due to noise from one antenna making its way to the others?
     
  5. SWL37632

    SWL37632 QRZ Member

    Many decades ago I built a 6 X 9 rotary switch matrix inside an aluminum box. Multiple antennas switched to multiple rigs with the option to 'short' the antennas to chassis ground.

    Although there was 'conducted' isolation (don't remember the actual numbers), the receive 'cross-coupling' was pretty bad....unacceptably bad when using large antennas.

    Later, I designed an R&D RF switch matrix for a Navy program (RX and TX) and used ITT vacuum latch relays for 2 MHz to 1 GHz. I had to use 2 in series to meet the required 'conducted' isolation.......

    Interestingly, I did a measurement a couple weeks ago of one of those ITT relays and found 'conducted' isolation of about 30 dB at 29 MHz and about 40 dB at about 500 KHz. From memory, the switch contact capacitance is about 4 pF.

    For receive, you will need to take care to shield each relay or switch to maintain the low 'cross coupling' to other relays or switch which is feeding other receivers.

    In an 'open frame' type of design like the one you have described, you should expect a high amount of this 'cross-coupling'.

    The alternative, to maximize receive isolation and minimize cross-coupling, is to use coaxial relays as already mentioned in a prior post.

    Good luck.

    Bill

    20+ WPM 1970's Extra
     
    Last edited: Nov 13, 2021
  6. KW4TI

    KW4TI Ham Member QRZ Page

    I played around with many relays for HF. Contrary to most accepted wisdom, definitely there are relays that are poor choices for HF.

    I would advice two types:

    For high power, use a Omron G2RL-1 or similar cradle-type relay. The HUI KE HK14FH is an economic alternative. So is the SMIH-12VDC-SL-C.

    For low power, the SPDT G5V-1 or G5V-2 are good choices. As an alternative for G5V-2, you can use Hui Ke HK19F.

    For those who wish to read my results with relays, you can consult:

    https://github.com/profdc9/ModularTuner/blob/master/doc/ModularTuner-Guide.pdf

    It describes some simple measurements one may perform to assess how well a relay performs at HF.

    73,
    Dan
    KW4TI
     
  7. SP4IT

    SP4IT Ham Member QRZ Page

    Dan and Bill, thank you both for sharing your experiences.

    Bill, could you share how you made the wiper in your custom switch? Or was it taken from another commercial unit?

    So as grounding unused rx antennas is definitely necessary this slightly complicates a wafer switch design. I would need to make my own switches as Bill did, or incorporate extra digital logic in the device.

    I've reviewed the relay types mentioned and I found I already have some that are very similar. The price of individual relays is not bad, but when one contemplates needing 30 for a project it becomes the main cost item. If this was the only way I would no doubt go down this path especially in light of information provided in the pdf Dan linked above (1.5pF to coil and 4pF between poles of stray capacitance for one example general purpose relay, if I remember correctly).

    So as in most of my DIY projects I attempt to find the most cost effective solution that still performs no worse. This brings me to those little glass tube reed switches. I can get them locally for about $0.5 each(0.3amp, 2amp ones are about $1 each), or I can order them from China for half that price. I bought some to test.

    The easiest way to control them is to 3d print a rotor with glued on magnets. This method will be used to passively ground all inputs by a turn of a knob when the shack is not in use.

    I also need a way to ground all rx antennas quickly on command. This will be controlled by RX/TX switches in all my rigs so RX antennas are grounded on TX. I need a way to activate the reeds with electricity, not just a turn of a knob.

    So I made a test coil with about 2ft of 0.1mm dia. (awg 38) magnet wire. It closes the reed switch with just 0.8V (about 120mA). So I'm getting thinner magnet wire (awg 44 - 0.05mm). A drop of super glue holds the coil together. Winding it takes few seconds when one spins the Reed switch slowly chucked in a cordless drill.

    Those DIY reed relays should be pretty good with regards to speed, pole to pole capacitance etc.

    Now I just have to decide if I want the main switching to be done with magnets, or electrically.

    73,
    Lukasz
     
    Last edited: Nov 14, 2021
  8. SWL37632

    SWL37632 QRZ Member

    Lukasz:

    I think you may have confused my inputs with those of KK4NSF.

    In my case I used standard ceramic commercial SP9T rotary switches, I did not build them from scratch as KK4NSF did.

    Please clarify your request.

    Just to further clarify, for transmit use, my solutions worked fine.

    However, for multiple receiver use, I determined that in my case, the 30 dB isolation and cross-coupling was not acceptable.

    Bill

    20+ WPM 1970's Extra
     
  9. SP4IT

    SP4IT Ham Member QRZ Page

    Yes, you're correct. I mixed your post with that of KK4NSF. Sorry.

    In the meantime I remembered the wafer switches used in Kenwood hybrid rigs use little rivets similar to those used for vias on DIY pcbs. So if I wanted to make a wiper the best way would probably be to make the wiper itself from copper foil and attach it to the pcb with a tiny tubular coper rivet. Copper work hardens easily so one could hopefully make a small piece more springy by hammering it.
    However, it is unlikely I'll be going down the custom switch route for this project as I managed to design simple transistor/resistor logic that does the job of holding all unused inputs to ground.

    Lukasz
     

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