The AM Forum

I need to build an ant relay.  What is a good guideline for the size contacts if they are going to survive 1kw pep on 160 meters with <2:1 swr at 50 ohms?
Were the old Dow-Key relays rated for a KW out?

The question hr is that I have some nice dpdt 24vdc relays but the contacts seem a bit small compared to typical big open frame contactor relays.

Tom
ILA

I found a HUGE 2P2T 110 vac antenna changeover relay, dating from the late 30's, at Dayton one year.  The thing has at least 1" spacing between open contacts, and the contacts are about 1/3" in diameter.  The insulation is ceramic.  But the thing makes a big "klunk" on make, and like any a.c. relay, it has an annoying buzz.

I decided to use it after it had been sitting on my shelf for at least 15 years, after I turned a dow-key literally into a block of charcoal with the Gates the first time I tried to  load it on 160.  I had a small wooden box with a hinged cover, so I mounted it on  the wall and installed the relay inside, suspended on spring mounts, with additional rubber shock mounts.  When I close the door, it is practically inaudible.

The movable contacts are attached to  the terminals with flexible copper braid.  Problem is, that braid wasn't flexible enough, and after a few weeks, it broke due to metal fatigue, causing me to lose contact with the antenna and blowing an rf ammeter in the transmitter.  I was able to repair it using some multiple-strand sub-braids from a piece of copper wire rope I picked up in some junk.  That copper is much more flexible, and after several years has never broken, although I periodically inspect it just to be safe.

This might work.. They're cheap.. And handle 40A. They come with asst. coil voltages from 12v to 120v ac/dc. Short parallel runs and hefty contacts
 Magnecraft Relay (http://www.alliedelec.com/Search/ProductDetail.asp?SKU=850-3013&SEARCH=&MPN=W199AX%2D13&DESC=W199AX%2D13&R=850%2D3013&sid=45F34680186DE17F)

Yes they do work,they are rated at 1500 volts open contact,I'm only running a Valiant through it and they are not noisy.I've had mine since 1984.Joe WB2FQD

Most slopbucket rigs (and leenyars, for that matter) just use off-the-shelf 4PDT relays.

My TransWorld solid state 1KW leenyar (good for 300W AM) uses just a plastic capped relay like that.

I used a P&B 25 Amp DPDT relay with both sets of contacts in parallel in the 4CX3000A rig. I ran the 120VAC coil on 28 VDC for years with no problem.

The name of the game with R.F. relays is to avoid hot switching them. Overkill isn't necessary. Beware of pre-owned relays. They may have been abused. 

I was using a 25-amp, P&B, DPDT self-wiping contactor-type relay with my 200W Class E transmitter.

One thing that I noticed was that the contact was unreliable in the receive position. I checked out the specification sheet for the relay... and, sure enough, there was a "minimum voltage" specification.

I could fix the problem, temporarily, by cleaning the contacts with contact cleaner applied to a piece of card stock (paper)... but the problem would reappear within a few days.

I don't know if this is a generic problem with high current contactor-type relays or not.

Stu

Stu,
I wonder if it is a used relay. The couple times I've noticed the problem all I had to do was key the relay a couple times to wipe the contact clean. P&B the contacts slide a bit when they make or break to keep them clean. A used relay with pitting may not be as good. Maybe try some 800 grit wet dry paper to burnish them .

I have seen this phenomenon in other pieces of industrial equipment.

Many or the power handling relays want to see a minimum voltage/current
being switched. This is to "burn" the oxidation off of the contacts. I have seen this with relays and microswitches that the contacts dont carry any current.

Like Frank says always look for relays and switches that have a "wiping" action of the contacts. This is always preferable, especially in low/no current switching situations. Sometimes a little bending of the contacts to create a wiping action will cure this problem.

                                                            The Slab Bacon

Best to avoid contact cleaner if these are silver or gold contacts....

Go to a jewelry store, and ask them for a jewelry cleaner that helps prevent corrosion or tarnish....

Though it might not matter, if the self-wiping action scrapes it all away anyway.....

Could run a dc voltage through the contacts in recieve mode using choke and diodes.


????

A match book cover is great for cleaning a contact in most cases.
Relay wetting current is usually low if the contact has never seen high current. A used relay with a couple arc marks will need a lot more wetting current. Dry switching a relay will guarantee a long life. Some new relays are rated at a million cycles if treated right and it can drop way down when wet switched.

For receive contacts, use the Ralph method. Current limited DC across the contacts decoupled through R.F. chokes. Visit quickdraw Ralph on the What's For Dinner Net for details. He has the fastest relays in the East.

There's a procedure in the Kenwood TS 430S manual to run dc thru the receive contacts for cleaning purposes.

Boy, aint that the truth!! For someome his age, he's got the fastest finger in the east!! And the fastest snipe/drive by on AM!!

The magnacraft W199AX-14 120VAC,w199w-12 12vdc seem to work good on receive with out any cleaning.I have the 120vac model and have never cleaned the contacts since the early 80's ,however  that might be the reason ,silver contact gold flashed,self wiping,I don't know if the new ones are built the same.Allied has the cheapest price for the 120vac 15.15 ,Newark is the cheapest for the 12dvc 18.74.Joe WB2FQD

High resistance make contacts is a well known problem on relays that have no current through them. The contacts oxidise, and you only need a very small layer to provide an insulator. The breakdown voltage of this insulating layer is low, so any significant voltage across the contacts will break it down.
The solutions are - wiping contacts that scrape the layer off as the relay closes, non oxidising contacts(expensive) or add 'wetting current' throgh the contacts. This was ALWAYs done with phone exchange relays and switches.
Ive had the probelm using AC power relays here and ended up doing something with resistors and chokes to get some current though/voltage accross  the contacts.
        Ian VK3KRI

Ian,
you remind me of the days of JS working for motorola.....