[NLRS] Dummy load

[Dr. Gerald N. Johnson]

A roll of Belden RG-58 works decently as a dummy load at 144 and higher. 
100 feet at 144 has enough loss that it hardly matters what you do with 
the other end, and its even better on the higher frequencies. On the 
higher frequencies the power loss will not be through the whole piece so 
it might be useful to drop the spool in a bucket of water for cooling. 
Keep the ends dry.

At 144 Mhz RG58 has 6 dB loss per 100 feet, so 12 dB round trip loss of 
the far end is open or shorted, and that's reflected power 1/16 of the 
forward power. SWR of 1.13:1. RG58 is only rated for 50 watts at 1 GHz 
so you might want to use RG-8 or 213 which the handbook shows about 9.5 
dB loss /100 feet at 900 MHz so the round trip would be 18 dB, a really 
nice SWR is possible.

Alternatively some MFJ tuners had a 6" 50 ohm globar that could make a 
decent load at VHF. MFJ-849 for one. The trick is for that is that the 
shield to resistor characteristic impedance needs to be the same as the 
resistance to the grounded end of the resistor. So the shield needs to 
fit the resistor snugly at the ground end and at the middle have a 
characteristic impedance of 25 ohms and at the coax end have a 
characteristic impedance of 50 ohms. That means a taper along the 
resistor that's logarithmic because characteristic impedance is a log 
function of the diameters (138 log10 (b/a)) and any sort of taper, 
usually linear from the fat end of the resistor to the coax connector. I 
have had a Narda 15 watt dummy load apart that is good to 12 or 15 GHz 
and that is how it is built with a metal oxide film resistor that has a 
ceramic core that extends past the ground end to the finned heatsink. 
One I have was broken and I was able to repair it electrically but not 
thermally, and it still shows a good SWR at 10 GHz.

www.ebay.com/itm/Non-Inductive-Resistor-50-ohms-Globar-50W-RF-Power-N-I-Re-Carborundum-SP-/221764846728?pt=LH_DefaultDomain_0&hash=item33a2370488

Surplus Sales of Nebraska has several dummy loads, some reasonably price 
surplus or commercial, some not so reasonable.
www.surplussales.com/RF/RFDummy-2.html

Fair Radio has a few loads, but probably none really good above a GHz.
www.fairradio.com/catalog.php?mode=view&categoryid=176

73, Jerry, K0CQ

On 5/7/2015 5:54 PM, Wyatt Dirks wrote:
>
>
> Was wondering if anyone might have a spare dummy load or maybe knew of a
> place I could get one.
>
> Need something good to 4ghz and if it could handle 100w that would good.
> Less might work as well.
>
> 73 Wyatt
> Ac0ra
>