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 Post subject: Improvised dummy antenna question
PostPosted: Jun Tue 04, 2019 5:44 am 
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Joined: Jul Sat 28, 2012 9:32 pm
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I have a knight kit T60 transmitter I'm going to be working on. It's a 60 watter and I'll be using a light bulb and porcelain socket as my poor man's dummy antenna. The only bulb I have laying around is a 500 watt photoflood bulb. Will this be ok?


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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Tue 04, 2019 5:56 am 
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Location: SW WA state
henry finley wrote:
I have a knight kit T60 transmitter I'm going to be working on. It's a 60 watter and I'll be using a light bulb and porcelain socket as my poor man's dummy antenna. The only bulb I have laying around is a 500 watt photoflood bulb. Will this be ok?


Henry,

It's probably too low in resistance... If HF AC resistance is about equal to DC resistance, then with the filament HOT you should have about 28 Ohms... However, a COLD filament will have a resistance a fraction of the hot resistance, so you'll be far from a 50 Ohm load.
Best of Luck!

-Tom


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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Tue 04, 2019 2:40 pm 
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Your answer made good sense. But by the same logic it would stand to reason that a light bulb would not be a very good dummy load at all, especially for CW testing, as it always takes a few instants to light up. Correct?


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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Tue 04, 2019 9:11 pm 
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Heathkit and Knight assembly manuals both showed the use of a 60 W light bulb as a dummy load. (For 6146 or 807 low power transmitters.)

What's the down-side on CW? The tube can take momentary load changes. Once the bulb lights up, there won't be much resistance change between dots and dashes!

If you need to take measurements (like voltage), you can hold the key down for a few seconds until things stabilize.

Rich


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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Tue 04, 2019 9:33 pm 
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Oh, lord, at first I read that as photo flashbulb! :shock: I have a little Johnson CW transmitter like the one I had as a novice. I have plenty of loads for it but at that time I had the ceramic light socket and a short coax cable leading to the transmitter and a 60 watter as the load. It worked well enough. Does not present the same load as an antenna but the PI network in the output was apparently versatile enough to match the off impedance of the lamp. Obviously it may radiate a little bit of RF as compared to a proper termination but hopefully not far. But probably enough to be heard a couple blocks away if anyone was listening. Am I the only one that hoarded some cheap 60 and 100 bulbs when they were on the way out? A 500 watter would have a very low resistance and also not give you the visual output indication like a more appropriate bulb does. Nothing that says you cannot get some of the low wattage bulbs and put them in series. They still make up to 40 watt or at least 25 for special fixtures. The grocery store has those, let alone hardware stores. Or I will meet you in the alley downtown with a 60 watt bulb in my trunk, but be ready to pay for it, and watch out for the green police! If you had a 50 ohm load you could go right from the load to a matched antenna and make little or no adjustment but with the bulb you will have to retune the final. I have seen resistors on ebay that could make a good load. Some odd looking things that are flat ceramic packages and mount to a heatsink. Wide band RF loads. Not too pricey.

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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Tue 04, 2019 9:44 pm 
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Quote:
Nothing that says you cannot get some of the low wattage bulbs and put them in series. They still make up to 40 watt or at least 25 for special fixtures.


Actually, you want them in parallel.

Rich


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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Wed 05, 2019 11:58 pm 
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These loads like this one are numerous on ebay and pretty cheap to boot. https://www.ebay.com/itm/50ohms-DC-3GHz ... SwyYFaOeKz
But, you better have a big honkin' heatsink for them I would think. That could be improvised from scrap aluminum or what have you. I might order one some time just for fun, as I don't really need it. IF it works right, it will be better than any old bulb. You could easily add an RF sample circuit and detector to drive a relative output meter, or calibrate it for watts if you have a reference of some sort.

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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Sun 23, 2019 11:45 pm 
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Too bad the mounting flange of those resistors is one of the leads- no way to easily series-parallel for more power. If you could find some where they had two distinct leads you could make an easy kilowatt load.

Scott


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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Mon 24, 2019 12:08 am 
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Are those Chinese resistors really rated for 250 watts? It sounds like wishful thinking to me. Given a decent sized heat sink, embedded in oil, would that work ok for 100 watts I wonder?

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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Mon 24, 2019 1:56 am 
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Phil Coe wrote:
Are those Chinese resistors really rated for 250 watts? It sounds like wishful thinking to me. Given a decent sized heat sink, embedded in oil, would that work ok for 100 watts I wonder?


That style of RF power resistor is available from a large number of producers and they will handle very large amounts of power as long as your heat sink can keep it within the design operating range and that is the rub. Power is specified at a device temperature max of 100C and most de-rate power handling capability to near zero as temperature approaches 150C.

The flange mount refers to the heat sink connection while the electrical leads are separate.

For most ham usage, a "Cantenna" style load is going to be a lot less troublesome unless you really do need ultra low reactance at UHF.

Rodger WQ9E


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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Mon 24, 2019 2:08 am 
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But one cold be stuck into a Cantenna, couldn't it?

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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Mon 24, 2019 2:47 am 
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Phil Coe wrote:
But one cold be stuck into a Cantenna, couldn't it?


You would need to heat sink it and monitor the temp at least initially to make sure your design provides the proper heat rejection. It is sort of like those conduction cooled tubes from Eimac in the 1970s, the tube itself is very compact but it has a small surface area and it needs a lot of heat sink area with very low thermal resistance from the device to the sink.

The stock resistor in the Cantenna is huge by comparison so transfer of heat through the oil was simpler while these new little resistors are more of a point source heat generator. You would also need to be sure that the package is suitable for oil immersion.

I suspect the best setup for these resistors is to mount them to a heat sink and provide forced air cooling rather than trying to immerse them in oil.

Rodger WQ9E


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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Mon 24, 2019 3:06 am 
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If I recall, the Cantenna used a 50 ohm carborundum resistor. I cooled mine with mineral oil.

https://www.rfparts.com/resistors/resis ... p500k.html

Image

Rich


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 Post subject: Re: Improvised dummy antenna question
PostPosted: Jun Mon 24, 2019 12:23 pm 
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Thanks Rich and Roger, etal.

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