I need help in designing an Automatic Tuning Unit (ATU) for HF (3-30 MHz)

Hi, Im a student and we have a project where we are designing an Automatic Tuning Unit (ATU) for HF (3-30 MHz) with an antenna whip 1.5 Meter. I am in charge in designing the ATU. Basically the ATU would have to good VSWR in each HF frequencies. i use 8 toroids with which have different value of indutance. we know that antenna whip with length 1.5 m have match frequency in 50 Mhz, so if we use that antenna for HF (3-30MHz), the antenna always have capacitive behavior. i do with linearity value of inductances for each of toroid, i set value 2x, so i get value of inductances for 8 toroids is 0.5 uH, 1 uH, 2 uH, 4 uH, 8 uH, 16 uH, 32 uH, and 64 uH, but there are a gap frequency. please help me, guys. I would greatly appreciate any of your assistance. Thank you very much in advance.

An inductor that can be rotated by an servo motor to add or remove more windings and in doing so change the inductance

Probably switching just different values of 8 inductors would not be enough to cover entire range, without gaps.
You might need to switch some tuning capacitors also.

Take a look to the schematics of SGC products, which for many years they design HF automatic antenna tuners:

http://www.sgcworld.com/PubInfoPage.html

As a first step, you would want to review expectable complex antenna input impedances from literature or measure them with a VNA, then calculate the optimal matching network for different frequencies.

You'll notice, that the radiation resistance at 3 MHz is extremely low and possibly can't be managed with a simple LC Pi-filter.

You should also check out the Racal ATUs (awesome UK designs), some screenshots are towards the end of this page:
http://members.chello.nl/~t.bloem/ham/HF/hf.html

thank you very much for your attention and your suggestion, but can u give me a specific reference for use a servo motor to add or remove more windings because i'm a beginner fot that application

thank you

---------- Post added at 11:09 ---------- Previous post was at 10:54 ----------

thank you very much for your attention and your suggestion, i'm use a whip antenna with length 1.5 Meter, so it's mean that whip antenna for all frequency HF for 3-30 MHz is still short, and always need an inductors not capacitors, may be i need a specific reference from

you to prove that i have to use a capacitor.....and i have look http://www.sgcworld.com/PubInfoPage.html, but over there written minimum antenna length is 8 feet or 2.6 Meter is not same with me because i'm use a whip antenna with length 4.5 feet or 1.5 Meter, please

give me a specific reference, for my final project, I would greatly appreciate any of your assistance. Thank you very much in advance

thank you

---------- Post added at 11:40 ---------- Previous post was at 11:09 ----------

thank you very much for your attention and your suggestion, i have read and looked that website but i don't know about antenna length of racal radio use, and i don't know about the result for each HF frequency, may be u can show me and give me an another specific reference, please help me to finish my final project .

thank you

That URL is for an ATU for HF frequencies, so you can get an idea of it from the screenshots (e.g types of relay required, approx size of coils, etc),
but unfortunately I have no info on it.
However, for a different Racal ATU, I found this doc via google, which has a very good description:
http://www.vmarsmanuals.co.uk/newsle...es/vra549c.pdf

Some tips: make sure that the RF doesn't get into the microcontroller! and make sure the coils don't
get damaged when hot when operating, another nice thing is to make it
automatically tune on a 'double-press', i.e. if it sees two quick pulses of RF, then it knows that the user has
pressed the transmit button twice quickly to signal an intention to tune, so it invokes the auto-tune function.
It means no button is needed to invoke the auto-tune.