Bad ohm problem?

[scubasky]

I am getting crappy reception on a Scantenna mounted on a 21' pole. The stock antenna is getting better reception. I took it down today and took all the measurements.
With a 75' run on rg-6 here are the numbers.
Outside shield on to outside shied on the ends of the cable gives a total of 1.3 Ohms
Inside conductor to inside conductor on the ends of the cable gives total of 2.5 Ohms.
From the scanner end of the cable outside conductor to inside conductor gives. 3.8-3.9 ohms
With both cables off of the antenna I tested them and there is NO short
The antenna is NOT grounded to the mast by touching anything, and all connections are tight.
The 50' run shows 3 ohms on it's own
The 25' run gives me .9 ohms on it's own

The attached picture is the setting my multi meter was set on if someone can verify that it was on the right setting (Ohm mode and max 200 setting)

 

[ReceiverBeaver]

Hey guy, You're on the right track but you're thinking at the moment of DC electric principles. In this case where you're essentially just checking continuity of conductor runs and the separation between the two. And this procedure is valid but now that you've tested your coax to be good....what next to look for to account for the poor reception?

Your antenna system, from indoor coax tip up to the top of the antenna, must be tested in a different way and as one piece to assure ordinary function and electrical receving match to your radio. Radio-antenna systems that we typically use are based around a 50-52 ohm impedance tuned circuit.

With transmitting stations like hams or CB'ers, SWR meters are common for testing resonance of which impedance plays a part. But the these meters require that a powered rf signal be run through them....which the radio does (provides a signal).

With scanners however you obviously have a receiver only. So there are a number of specialty test meters which have been marketed at the ham market mainly. they work really well, have been around for a good number of years now and are quite popular. A great one for us is made by MFJ and is their model 269 Antenna Analyzer. They combine a rf signal generator, frequency counter, and other componentry to create a antenna test meter that simutaneously generates a test signal for any desired frequency within its range, and displays SWR and Ohms Impedance at the same time on dual meters. This model operates from like 1.6mhz to 174mhz strait through, and then has a UHF range that is from around 420 to 470mhz. So this unit essentailly covers all HF, VHF and UHF common frequency ranges.

I've had one of these units and they're the best thing since sliced bread. It will possibly take something like this to analyze your problem (with a test instrument) seeing as everything else looks good to you. You do apparently a problem in some part of your system.

 

[scubasky]

Thanks. What kind of ohms range should it be in?

 

[LarrySC]

Another idea: You failed to mention the type of scanner you are using. This in it-self could be a factor. The GRE PRO-series R/S sells have a VHF overload issue when used with LARGE antannas. Also the balun could be bad. Must have a reading across all 3 leads using "R" - "Ohm" mode of volt meter. Wire lead to wire lead / each wire lead to "F" end. Good Luck.

 

[jim202]

Go back to your original meter measurement. You indicated that from the
shield to the center conductor at the scanner end you saw 3.8 Ohms. Was
this with the antenna connected? You did say that the cable showed open.

Let's try this again. With nothing attached to the coax, put your meter on
a high Ohm scale (megohms). You should not see anything. If you see
anything, the coax cable has leakage.

Check the antenna again. I am not shure if that antenna is DC grounded
or not. If it is, then you will see the short from center conductor to the shield.

Try another antenna on the end of the coax. My bet is that you have a
problem with the antenna. The connector might be shorted.

 

[mancow]

The scantenna uses a balun. It's a transformer. It's a dead short to DC.

Am I reading this right?

With the antenna connected and you in the shack you test the resistance between the inner conductor (center pin) and outer conductor of the coax. You are getting low resistance in the order of 2 to 3 ohms.

With the antenna not connected you are seeing no continuity at all (no connection) between the center pin and outer conductor from the shack side of the coax.

Is that correct?

The balun transforms the RF energy to match the impedance of the feedline at the antenna end. The radio signals can traverse the balun and make it down the coax. When you are trying to test it with DC voltage from a meter the voltage is running up the coax through the windings of the transformer and back down again. It's seeing it as one continuous shorted piece of conductor.


In other words, it's doing exactly what it should.

 

[mancow]

Also, with all the hoopla about the scantenna I read I decided to get one not long ago. I'm not all that impressed. Mine is up at about 25 feet and is connected to the supplied run of coax they send with it plus about a 6 foot piece so it's not that different from your setup. It works great for lowband. Vhf is fair, Uhf and above it's marginal at best.

It's not the whiz bang that everyone seem to say it was.

I think it's just the way it is.

 

[scubasky]

Yes it has a balun. The scanner is a Radio Shack pro 2054.

When inside with the antenna attached center pin to outer conductor gives 3.8 to 3.9 ohms.

When I put it to Mega ohms I get no reading center to outiside conductor

With the cable off of the antenna there is no continuity between the inner and outer conductor indicating NO short.

Testing the ohm load from one end of the cable to the other i am getting 1.3 on the outer conductor and 2.5 on the inner.

 

[Mischief810]

With a 75' run on rg-6 here are the numbers.

RG6?

This may be a really dumb or completely irrelevant observation, but isn't that the stuff you use for cable and satellite TV?

 

[N_Jay]

RG6?

This may be a really dumb or completely irrelevant observation, but isn't that the stuff you use for cable and satellite TV?

And any low power/low loss wide-band application.

 

[prcguy]

Mancow is right, most TV baluns will show a short at the 75ohm side. If you take the 1.3 ohm you measured from shield to shield at each end and add the 2.5 ohms you measured from center to center, that adds up to the 3.8 ohms you measured when the balun is connected. Everything DC wise looks good.
prcguy

I am getting crappy reception on a Scantenna mounted on a 21' pole. The stock antenna is getting better reception. I took it down today and took all the measurements.
With a 75' run on rg-6 here are the numbers.
Outside shield on to outside shied on the ends of the cable gives a total of 1.3 Ohms
Inside conductor to inside conductor on the ends of the cable gives total of 2.5 Ohms.
From the scanner end of the cable outside conductor to inside conductor gives. 3.8-3.9 ohms
With both cables off of the antenna I tested them and there is NO short
The antenna is NOT grounded to the mast by touching anything, and all connections are tight.
The 50' run shows 3 ohms on it's own
The 25' run gives me .9 ohms on it's own

The attached picture is the setting my multi meter was set on if someone can verify that it was on the right setting (Ohm mode and max 200 setting)

 

[Chevyman22360]

Try this once

Scubasky,
I have two scantennas and I am like you I am not impressed at all with the performance compared to what has been said on this group. I have tried different cables, different baluns different scantenna and still get same result everytime. Where you connect your balun to there are two tabs just opposite of the the wingnuts you tighten for the balun turn those two tabs so they touch each other and see if it improves your reception, it did mine alot but still not good enough to allow it to be up there. I have mine on a 60 ft tower. Let me know if this works for you!!

 

[scubasky]

Figured it out. If I turned on the ATT I get the signals fine. I live about 400 yards from a 240 foot communications tower at a electrical plant, and this seemed to fix my reception problem!