Dummy load bank

[patracy]

So I'm picking up a legal limit plus amplifier soon. But I have nothing to handle the load. Until now. I was given a 1kw AM transmitter load bank from a fellow ham. Ive read about the old heathkit cantenna dummy loads and decided to make something along those lines from it. I picked up a PVS-14 storage "ammo can" that fits nicely. I'll be filling it with transformer oil next week. (New stuff, not pcb). But I'm pretty much done with it now. Nice 50ohm load. Added a stencil to it and another coat of OD. I stated 2kw, based on the factoring I saw of the submerged cantenna design. Anyway here's a few pics.

 

[swbradley1]

Nice work Drew.

 

[steelandcanvas]

What is the value of your resistors?

 

[tim292stro]

I'm going to guess somewhere in the neighborhood of 300-Ohms... 500Watt each Patracy?

 

[patracy]

What is the value of your resistors?

300 Ohms. 200 watts each. So wired as they are, 50 ohms and 1200 watts to air. The submersion in oil allows you to cheat in a sense. The oil is a dielectric and a heat conductor. So you can easily double the capacity, almost triple it. (Depending on the oil, mineral oil is only about double, transformer oil is triple) I've given the rating the "military" style. Take what it should handle then reduce it by 33%. Also instead of only a gallon, this setup will hold 3 gallons of oil.

I'm picking up a Henry 2KD Classic linear. I know going in the tubes are soft in it, it's only putting out about 800w. But I plan on replacing the tubes to get it back to legal limit+. So with all of that, I know the dummy load could handle the full output of the amp for at least 10-15 minutes. The limitation comes with the oil at that point. It works as a bell curve.



The heathkit cantenna has 1 gallon of oil in it. And the resistor was 500w. (At least by the accounts I've found, lot of it is speculation) Notice the two curves as well given the oil types. Once I get everything setup, I'll try to measure the temps/ohms over time to develop my own bell curve for a operating placard.

 

[Flyingvan911]

Very nice!

 

[patracy]

Nice work Drew.

Makes sense now doesn't it with the WTB ammo can thread huh?

 

[swbradley1]

Yep

 

[m16ty]

From somebody that barely knows how to wire a CB radio into a truck, what is this load bank used for and why is it needed?

I've dabbled in CB linears and we never used anything like this running some pretty big linears.

 

[tim292stro]

When testing a radio set, you don't want to run a full antenna as you could be broadcasting noise or garbage. You also don't want to have an unloaded output, or you'll probably blow your output stage. Best way to test is to have a dummy load - it simulates the load of an ideal antenna system, without radiating RF energy out into the world (it's confined in the "can").

 

[steelandcanvas]

I thought I would toss this in the thread: I am presently studying for my Technician License, something I have wanted to do since my teens.

 

[patracy]

I thought I would toss this in the thread: I am presently studying for my Technician License, something I have wanted to do since my teens.

Hurry up and get that ticket! I'm up to a general now. I'm about to start studying for my extra. I honestly had planned on getting General at most. But I hear so many neat conversations on HF that are in the extra band segments, that it made me want to go all the way. Couple of days ago I was listening to a conversation between two operators in German. I didn't catch the call sign of either, but I heard mention of a few towns I know are in Germany. Either they were both in Germany, one was in Germany and the other elsewhere, or both were from Germany and perhaps stateside. Then a month or two ago I heart a conversation between an operator in Denver, CO speaking to someone in Sidney Australia. I could hear the Denver operator clear as day. The Sidney operator was about a 2 over 3, but it would have been enough had I had a headset on to carry out a conversation with. Bear in mind, Denver is over 1300 miles from me and Sidney is over 9200 miles away!

 

[swbradley1]

I thought I would toss this in the thread: I am presently studying for my Technician License, something I have wanted to do since my teens.

Yes, by all means get your ticket. Even though I got all three of mine in a month I have only used them at the GMVR last year and this year. The only person I talked to was patracy when I forced him to do a night ride in his HMMWV.

 

[papakb]

I wanted to toss this into the ring since we're on the subject of licenses. There are a lot of MV owners out there with radios in their vehicles but I wonder how many are FCC licensed. There's no license to operate them in receive mode (unlike the Brits) but if you want to transmit you should be licensed. A simple Technicians license will cover you to transmit in the 6 meter amateur band (~50-54 mhz). The hams don't particularly like military radios because the majority of them are wide band radios and most of their stuff is narrow band but the 6 meter band is not that popular and since our radios are low power we generally don't disturb them too much. The typical VRC-12 or Sincgars sets will operate on a much larger frequency range but if you stick to the 6 meter segment and keep the idle chatter down no one will complain. Use common sense and remember these things aren't the CBs of old. If you have to chatter incessantly buy a cheap set of FRS radios.

Kurt
KG6KMJ

 

[patracy]

Actually a tech license will also let you into a lot of the other bands. But when it comes to voice, you can only get into 10m. Everything below that is CW only.

 

[steelandcanvas]

Hurry up and get that ticket!

Yes, by all means get your ticket.

Thanks for the support! The testing may not come as soon as I desire, as I am also studying for a couple of work oriented certifications...all in good time though.

 

[T. Highway]

Drew,

Question, aren't you concerned about the crimped connections loosening over time from the heat / cool cycle?

Bert

 

[tim292stro]

A proper crimp job creates a metallurgical bond between the connector and wire:

 

[patracy]

Drew,

Question, aren't you concerned about the crimped connections loosening over time from the heat / cool cycle?

Bert

Those crimps, except for the ground and antenna leads I made, are the same wires/crimps it's had over the years. I'm not sure how many decades old the bank (aircooled) is. But it still ohms out at 50ohms. The 239 connection is soldered together.

 

[T. Highway]

A proper crimp job creates a metallurgical bond between the connector and wire:
View attachment 598168View attachment 598169

I fully understand machine crimped terminations, that encompass all sides in mandrel dies. Hand crimped terminations are a whole different animal.

That's why I ask.

Bert