Antenna tuning question ?

[Skypilot007]

I have a Kenwood TS-440AT with a diamond SX-200 external power/swr meter and an I-max 2000 10 meter antenna. The radio also has it's own power/swr meter built in. When I tune the antenna with the built in antenna tuner in the radio I get different swr readings on the internal and external meters. I always just went by the meter in the radio and had good results so I thought but decided to add the external meter figuring it would be a little more accurate and easier to see. I used a scrap piece of the same coax thats feeding the antenna to make the jumper to connect the external meter and its about 2 ft in length. I've double checked all connections on the antenna and jumper and everything is good and the antenna is well grounded.

So the question is which meter do I believe?

or...

Do I have an antenna or feed line problem?

Any advise appreciated.

 

[prcguy]

The meter in the radio is looking at the inline meter and extra coax, so the readings may be different. Neither meter is a piece of lab equipment and they both could be off by quite a bit.
prcguy

 

[rfradioconsult]

Go with the internal meter. It is located after the tx finals and prior to the tuner/impedence matcher. The complex impedence of the antenna & feedline is matched by the internal tuner. The external meter becomes part of the feedline and is not treated as a 50 ohm device. Consider this; if you were using a radio in conjunction with an external VSWR meter and external tuner you would connect the meter between the radio and the tuner, not after the tuner. Good luck.

 

[prcguy]

I missed the part about internal tuner. rfradioconsult is right, the external meter will not show what the radio is seeing before its internal tuner.
prcguy

Go with the internal meter. It is located after the tx finals and prior to the tuner/impedence matcher. The complex impedence of the antenna & feedline is matched by the internal tuner. The external meter becomes part of the feedline and is not treated as a 50 ohm device. Consider this; if you were using a radio in conjunction with an external VSWR meter and external tuner you would connect the meter between the radio and the tuner, not after the tuner. Good luck.

 

[k9rzz]

Worded differently ... the external meter indicates what your antenna system actually is, without any tuner. You may still need the tuner to get your transmitter at full power though.

 

[Skypilot007]

Thanks for the advise guys. I think I'll just lose the external meter for now and save for another set up.

 

[k9rzz]

Sounds like a plan.

Think of it this way. What you have:

TX -> SWR Meter -> Tuner -> SWR Meter -> Antenna

:^]

 

[kb2vxa]

Let's put it this way, the external meter is reading the true antenna SWR while the internal one is reading what the tuner fools the transmitter into what it thinks it should be. If the ATU is working properly you'll always have a good transmitter (not antenna) match no matter what the other one says. If the external meter reads good (less than 2:1) forget the tuner, switch it out.

If you have a resonant antenna your SWR will be good without a tuner, perhaps you may have to tweak it a bit at the band edges but that's about it. High SWR means high return losses regardless of any tuner. All one does is satisfy the transmitter's 50 ohm requirement, you'll still not be radiating all the power you're putting into the antenna but wasting it heating up the tuner.

 

[key2_altfire]

Here's another one for you. I read on Wikipedia that an SWR meter is best used as close to the antenna as possible. After I thought about it, it made sense. If the SWR meter is used at the antenna end, the closest match to the antenna is made as the RF exits the coax. This way you are measuring reflected power (e.g. mismatch) at the RF radiator. Match at the transmitter is of no consequence as long as it is in a safe range for the PA final.

 

[jonny290]

or you can cut to half-wave multiples on your coax (adjusting for velocity factor), thus the impedance seen at the antenna base will be mirrored at the shack end of the feedline.

 

[N1508J]

Radio sentience????

Let's put it this way, the external meter is reading the true antenna SWR while the internal one is reading what the tuner fools the transmitter into what it thinks it should be. If the ATU is working properly you'll always have a good transmitter (not antenna) match no matter what the other one says. If the external meter reads good (less than 2:1) forget the tuner, switch it out.

If you have a resonant antenna your SWR will be good without a tuner, perhaps you may have to tweak it a bit at the band edges but that's about it. High SWR means high return losses regardless of any tuner. All one does is satisfy the transmitter's 50 ohm requirement, you'll still not be radiating all the power you're putting into the antenna but wasting it heating up the tuner.

While the intent is to provide a simplistic explanation of the transformation taking place in the matching circuit, the implication that the circuit pulls a fast on the final is deceptive. The matching circuit does not "fool" anything, nor does the final "feel" embarassed having been suckered by the matching circuit.:lol: Otherwise, a very accurate post!:wink: