YOUR RF & MICROWAVE TECH Q&A RESOURCE

You may know Harlan Howe from his twelve years as publisher and editor of Microwave Journal ®, or from his 34 years as a Microwave design engineer and engineering manager, or from his service as an IEEE fellow and past president of MTT-S.

Now, although semi-retired, Harlan is available to answer your questions about RF and Microwave engineering. If he doesn't have the answer, he will find an industry expert who does.

FROM: Robert Green, Woods Hole Oceanographic Institute.

Dear Harlan,

I am currently designing an underwater radio for Alvin, the deep manned submersible. For the first phase of this design I used an off the shielf transmitter receiver module. My problem is designing a mono-pole antenna at the proper wavelength. Do you, or any of you readers know how salt water effects the wavelength of electromagnetic waves? I have searched and searched technical books to no avail. Any help would be valuable.

The first radio frequency is 315 Mhz

The second radio frequency is 31 Khz

Thanks,

Robert Green, Woods Hole Oceanographic Institution

--------------

Dear Robert,

The wavelength is shortened by the square root of the dielectric constant of the medium of propagation. The dielectric constant of pure water is about 78. I'm sure salt water is even higher and probably varies with depth and location. I'm not aware of any specific texts, however, a book on Ocean Engineering will probably have some material data.

FROM: Kartikey Parmar, Charotar Institute of Technology

What is YIG (yittrium iron garnet) filter? What are its design considerations & applications?

--------------

Dear Kartikey,

A YIG filter is a tunable filter based on the resonance of a sphere of YIG material which is tuned by changing the magnetic field around the sphere. They are widely used as tuners for wideband front-ends since they will tune over an octave bandwidth. It is possible to design multiple pole filters by using more than one sphere. One drawback is that magnetic tuning is slower than semiconductor tuning and special circuits are needed to compensate for hysterisis effects.

FROM: Keith Alexander, IP Access Ltd

Hi Harlan,

Tuning the o/p of a PA for optimal P1dB is relatively quick and easy using a triple stub tuner.

But in the absence of load pull contours from the maufacturer,trying to find the optimal OIP3 match by the same method is painfuly slow as there are so many operations neccesary to make each measurement.

Apart from the obvious solution of buying an expensive load-pull set up, do you know a quick(ish) technique for homing in on the optimal match ?

Many thanks

--------------

Dear Keith,

I don't know of any quick way except for the load-pull test sets that you mention. As you have observed, the manual methods are all very tedious. Although they are expensive, load-pull test sets pay for themselves very quickly in terms of reduced labor and increased accuracy. They also reduce the possibility of missing some state due to using discrete manual steps.

FROM: John Mehr, Ball Aerospace & Technologies Corp.

I am designing a stripline matching board for a UHF antenna with a 100 Watt CW input power requirement. The substrate materials are 0.031" thick Rogers 3003. Also, the antenna is not well matched over the entire bandwidth, and there is a possibility of having standing waves on the matching board. Could you tell how to calculate the minimum trace width & thickness that I can use in the matching board?

--------------

Dear John,

You don't say what impedance line you need to use for this matching, so I can't give you a specific answer. The equations for impedance vs. dimensions of the line are given on pages 33 to 39 in my book: "Stripline Circuit Design ", Artech House, 1974. A discussion of power handling capability is given on pages 17 to 31. If you are working at lower frequencies, that will permit it, you may want to increase the board thickness, thus linewidth, to improve the power handling ability.

FROM: Duncan Simpson, Simpco. Maint.

When being used as a weapon what are effective was to block or jam incoming signal?

--------------

Dear Duncan,

It depends on what you mean by "used as a weapon". If you are talking about radar, missile or armament guidance, there are many deceptive jamming techniques currently in use. There are also signal processing circuits to defeat noise jamming. If you mean "HPM (High Power Microwave)", then shielding or reflection are appropriate. In the case of HPM pulses designed to burn out circuits, filters, plasma switches, limiters and absorbing materials will help, however, if you mean high average power as in the ABM program, there is little that can be done.

FROM: Hosain Menshady, IEI

Hello

1-I want to know about advantages and limitations of using millimeter wave Radio-Relay in Military Communication.

If possible, please introduce me to some millimeter wave Radio-Relay manufacturers.

2-After evaluating of some spread spectrum radio relay systems that I studied, I observed that almost all of them use FSK modulation and frequency hopping technique (slow & fast). Is there a special reason for this?

I am looking forward to hearing you.

Best Regards

--------------

Dear Hosain,

The advantages are narrow beamwidth and limited range, which make inteception more difficult. The disadvantages are the same. An additional advantage is increased information bandwidth. I am unwilling to recommend specufic manufacturers of commercial products in the column, however, you can check our manufacturer's directory on this website.

I am not an expert on spread spectrum modulation, however, I suspect that the reason is that FSK is the easiest and least expensive modulation technique, which lends itself to the spread spectrum environment.

FROM: SANJEEV GHILDIYAL, HIMANCHAL FUTURISTICS COMMUNICATION LTD.

I'm looking for 4-Bit MEMS Phase shifters in 9-to-10 GHz and 16-to-18 GHz Range. Can you suggest to me the available sources for these or the foundaries which do this work on commercial basis.

Regards
Sanjeev

--------------

Dear Sanjeev,

I am unwilling to make specific recommendations for commercial products in this column. However, we have published numerous articles on MEMS in recent issues, with credit to the author's company. You can also check the manufacturer's directory on our website. Sorry I can't be more specific.

FROM: Jon Tallbacka, Naviair

Hi Harlan,

In my line of work (radar) I have stumbeled across a manufacturer who claims that "the length of a waveguide from the radar antenna to reception have effect on the close range detecting a target or processing video." Frequency is 10 GHz. In my theory book three major topics have to be addressed: Zo, Vacum and smoothness of assemblies. If the aforementioned topics have been covered well you should be able to have v=c, vswr<1.1 and low refraction inside the waveguide. Where does the range of reception come into account?

The manufacturer has proposed to install the Tx/Rx inside the antenna mast to get closer than 15m of the antenna, thus having a better close range detection.

Do you have any comments or possibilities why it is necessary to decreese the waveguide length.

All the best,

Jon Tallbacka

--------------

Dear Jon,

You don't say how close you want to detect a target. If it's a mile, then there is no advantage. If it's a few meters, then the length of the transmission run is a factor, although that should be compensatable with a computer.

FROM: Keshav Raj, Indian Navy

I have equipment (Russian) for checking the quality of CW transmission in respect of amplitude, phase and frequency fluctuations. For measuring the frequency fluctuations it modulates the received signal at 3,10 & 30 Khz (at the input of the equipment). I am trying to replace this vintage equipment with a modern one but i am not able to find the reason behind the requirement of this modulation.

Also could you tell me what are the parameters which qualify the CW transmissions to be stable in terms of presence of noise in amplitude/freq/phase.

Thank you

Raj

--------------

Dear Raj,

I don't know what measurement principles are being used in your equipment, so I can't comment on the reasons for the modulation. Any modern spectrum analyzer should be usable for the measurements that you seek. There are many manufacturers in the US and Western Europe. Stability is a function of the type of source being measured, so there is no universal answer as to the effect of noise.

FROM: Xuesong Chen, Agilis Communication Technologies Pte Ltd

My question is about how to detect returned power. In my project, I need to design a high power solid state power amplifier.

At the output port I want to protect the last stage of amplifier against big returned power. So what kind of power detector can be designed? I know there is some isolator with returned power detect function in market but it is quite expensive.

Actually I have a forward output power detector which is a microstrip coupler. Can I use an reversed microstrip coupler to design the returned power detector?

Thanks and best regards,

Xuesong

--------------

Dear Xuesong,

A reverse coupler is the normal approach, however, it will have to have enough directivity to keep the reflection detector from being loaded by the leakage power from the amplifier. It's hard to make a microstrip coupler with more than 20dB of directivity. This may not be enough for your application.

FROM: Kah Boon Loo, Agilent Technologies

Hi Harlan,

My name is Loo, I am the product engineer at Penang. Before I start to ask any question, here is the summary for my problem.

- I am taking care of a product called Converter. This converter is used to down convert RF freq 3.9 GHz to 300 MHz IF. It contains single mixer with cavity filter that perfoms the mixing process. Recently, this product is failing TOI issu (3rd order interception). The analysis showed that it [would have] passed if the cavity filter was channged.

Below are my question:

-Any comments on the filter that contribute to the TOI issue?

-I understand that it might due to low LO power, but the power had not changed. Do you have any experiences/knowledge on this issue?

- What is the factors that will contibute to the TOI performance? (LO power, diode mixer)

- Any good advise on the troubleshooting of this TOI issue?

That's all for my questions. Thanks for your time.

--------------

Dear Loo,

I suspect that the filter is reflecting some spurious out of band product back into the mixer. Try adding a Ferrite isolator between them.

FROM: Enrico Lia, Huber+Sunher Latin America

Dear Mr. Harlan,

I simulated a microstrip hairpin bandpass filter (Fc = 1785MHz, 170MHz Bw) reponse with very good results over a standard FR4 substrate. Once I made it real, I found mounting a FR4 PCB with proper design the results was not the same. I barely could see the curves.

Can you help me to overcome this mistake? or is it a failure on CAE?

Please let me know what you can do for me.

Thank you in advance,

Enrico.

--------------

Dear Enrico,

The problem is probably related to your use of FR-4. This is not an appropriate material for a narrow band filter at that frequency. In addition, the dielectric constant of FR-4 is not well controlled. Try building it on one of the many Teflon-Fiberglass substrate materials that are available.

FROM: Mark Looney, Analog Devices

Mr. Howe:

Do you know of a good online resource for getting familiar with common RF amplifier terms/characteristics such as Noise Figure, IP2/IP3. I am looking for literature that will help my equate IP2/IP3 information to anticipated distortion in a single-tone application. Thanks

Sincerely,

Mark Looney

--------------

There are dozens of books on the subject, however, I am not aware of any online source of the specific information you seek. You can check out a number of book titles at www.artechhouse.com.