Thread: Second newbie HF question

Sometimes they can be a bit funny on some channel's due to a number of reasons out of your control. Out of interest try a different length antenna or remove the spring and attach the wire whip direct to the top of the tunner and try again.

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Alrighty then POD,

First a couple of questions for you.

Is the Moonraker mounted well on the vehicle? That is, good electrical connection to the bodywork and the top section and whip of the tuner clear of any nearby body work. Has the coax lead been shortened and/or re-terminated?

I ask this because in the case of the first the autotune units are VERY sensitive to the ground provided to them.

In the case of the second the length of the coax has a significant effect on the ability to tune certain frequencies if it is too short (they usually come in a minimum 6m length) AND if the tuner does not have a good ground.

Hope that helps some.

Cheers,
Iain

...........another item could be the Whip that came with the tuner. If the whip is the wrong length or has the wrong taper ( yes the SS whips do have taper) the auto tune may well throw a hissy fit .

Cheers

RF

RF

Thanks for the responses.
The antenna is earthed well both to the chassis and to the big aluminium box on the back of my vehicle (see my signature photo)- I figure that is essentially the body.
The top half of the autotune base, and obviously all of the whip, is well clear of the body. The moonraker info says that just the fibreglass tuning section of the base needs to be clear of bodywork- no problems there.
I have removed the spring base from the whip, has not made any difference.
Yes I have re-terminated the coax, the section from the radio to the antenna control unit is about 250mm long, and the section from the controller to the antenna is about 2m long. Is this likely to be contributing to the problem? The thing that really puzzles me is that, except for the couple of times mentioned in my original post, the antenna does not even attempt to tune on channel 4. Just gives me a UE message and error tone as soon as I press the 'tune' button.

Hmmm,

Sounds like you've done as well as you can to ensure a good RF earth for the antenna. I can't think of anything further to suggest on that score.

I'm not sure how the Moonraker operates to tune but I suspect it's not much different than any of the other tuners out there - it tries to make the impedance match (via various combinations of inductance, capacitance and antenna length) as close as possible to ideal to reduce the SWR. This process does mean that the length of the coax does have an impact on the ability to tune different frequencies. If you have a longer feedline (6m or more) it may be worth a try to see if it makes some/any difference.

As to the attempt or not to tune, this could be due to a number of factors. Some radios remember tuning settings, some tuners remember their own settings, sometimes an interface unit has a memory. Also if the SWR was extremely high at first attempt to tune, most radios shut down to avoid damage and they won't attempt to continue the tuning process. This may be what's happening in your case, or it could be something else entirely. Hard to say by remote diagnosis unfortunately.

My 8525B has trouble with Channel 5, sometimes it tunes and sometimes it beeps. In my case the earth is dodgy between antenna tuner and car, so coax length has much bigger impact on the tuning process than if the earth connection was better.

Ah, also forgot to mention when talking about the antenna, the body of the radio also has a good, large earth point on the back of it. Make sure this too is attached solidly to the body work via a nice large piece of wire, or even narrow braid. That too has an effect if not sound.

That's all I can think of just now. Keep hunting. Like the truth, the answer is out there.

Cheers,
Iain

Okay, will improve the earthing of the radio and the antenna control unit, I have them mounted to metal bodywork but no dedicated earth straps.