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how to read ohms on coax??

B

bluestallion

W9WDX Amateur Radio Club Member
May 13, 2008
151
8
28
I'm going to be getting some hamsticks soon and want to make sure they are matched 50ohms to my coax. How can i check this without an antenna analyzer? Can i use a multimeter? thanks
 

A DVM can only read resistance. Impedance is pretty much resistance when mesured with AC. RF is AC.

Use an SWR meter. It will tell you what you need to know.
 
Robb said:
I would use a barrel connector and a 50 ohm dummy load at one end of the coax and an antenna analyzer like a MFJ-259 and test its impedance that way . . .
Click to expand...


Huh?

That wouldn't tell one anything about the impedance of an antenna. Only the feed line.
 
He asked if the antennas were also 50 ohm. Not the feedline.

Many times an impedance match does not mean an antenna match.

Only reflected power is important. That's why SWR is what he needs to pay attention to.
 
was just wondering cuz i see some pictures of guys that have little matching coils on their hamsticks to bring match of the antenna and coax to 50 ohms. I didnt know if this was all important or not. I dont have an analyzer yet, but would love to have one, just didnt know if could do same with meter or not.
 
Bluestallion, mount your hamsticks. The use an SWR meter inline.

Adjust the whip on the hamstick to get the lowest SWR reading possible for the band it will be used on.

Many HF SWR/power meters are fairly cheap, I like the MFJ's. On one of those you adjust to the lowest reflect power.


I use the 815B, a cross needle unit. The place the needles cross gives you the SWR. One gives forward power and the other gives reflected power.

The new one is the C model.

MFJ Enterprises Inc.
 
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What radio are you using and where are you mounting the hamsticks?

Mobile or base?

Lots of variables.
 
Nice rig, I had one.

The SWR meter on it can be trusted. If the SWR is too high, it will lower the output power to protect the PA.

What bands are you wanting to operate on?
 
If you have a good clean installation then the chances are your hamstick will have a low feedpoint impedance, this is why you have to add shunt inductance to the base of the antenna in order to match it to 50ohm.
With a shortened antenna you have to tune the antenna for resonance, and then tune it for low VSWR, the two don't necessarily coincide, they certainly don't on my homebrew 8' mobile antennas, my 20m version requires around 8uH shunt inductance for a low VSWR, however my stubby screwdriver antenna requires none at all, guess which is the most efficient?

Of course this is band dependant and the more the antenna becomes a significant part of a 1/4WL then the less shunt feeding is required, as someone said previously, lots of variables, and you may need no shunting at all dependant on band, antenna performance and antenna location ;)
 
You can also look at the MFJ antenna tuners auto or manual that way you have a tuner for fine tuning and will also have the nice crossneedle forward and relflected SWR / power meter all in one. They make some nice mobile tuners at reasonable prices. I have 9 antenna tuners with 6 of them being MFJ tuners 1 kenwood and 2 Yaesu and 5 little antenna matchers but I want one of these new MFJ mobile tuners. They are reasonably priced and a 300 Watt 160 - 6 meter capability with an antenna Tune / Bypass switch for $129.00



MFJ Enterprises Inc.
 
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