Expert: Chris Bushman - 2/10/2009

Question
QUESTION: First, let me say thank you for being available to answer the sometimes inane questions of novices such as myself.

So here goes, I have a shortwave radio (Grundig YB-400PE), an old Midland CB radio (40 channel) and I hopefully will be getting a radio scanner soon.

What I would like to do is to install all 3 pieces of equipment in my shed and use them individually when time permits.  Can I install one antenna which could service them all effectively.  This would of course require a splitter of some sort so I wouldn't needed to be constantly pluging the antenna in/out of each unit.  I would only be using them one at a time.

Any suggestions on an antenna that could do this?  I was planning on getting mobile whip antenna and installing it on a 10 foot pipe in my backyard.  I know that sounds kind of hokey but I would like to do this as cost effective as possible but still be effective.

Thanks again for you time in answering my questions.

Jim Kempf

ANSWER: Jim, I admire your innovative and creative spirit. No question is unworthy for people like you.

Here's the general rule of antennas. The optimum size of an antenna is determined by the frequency (which corresponds to the wavelength). Higher frequencies have shorter wavelengths and lower frequencies have longer wavelengths.  An antenna will work best for only one frequency.

The three radio systems you mention cover three very different frequency ranges. Shortwave receivers are MF and HF (Medium Frequencies and High Frequencies) in the range of 300Khz to 30Mhz. CB radio is at the top end of that range at 29Mhz. Scanners usually cover the VHF & UHF (Very High Frequency and Ultra High Frequency) bands which range from 30Mhz to 3Ghz.

One type of antenna called a quarter wave vertical would be around 40 feet tall for a freq of 7Mhz, around 9 feet tall for 29Mhz, and 19" for 144Mhz. Up around 3Ghz we're talking parabolic dish antennas like for satellite TV.

But the good news for you, since you are (mostly) not transmitting, antenna size is not super-critical. If you were to transmit using a grossly mismatched antenna, it could blow up you transmitter.

So what you really need to do is get some metal up there that can best suit your needs. Since you might be transmitting on CB, do exactly what you suggested and throw up a simple mobile CB whip antenna (without coils). It will work well on 29Mhz and good enough on the other freqs.

Get yourself a coaxial antenna switch so that you can use the one antenna for all 3 radios. Something like...

http://www.universal-radio.com/catalog/switch/0362.html

See, that wasn't a hokey idea at all!

Chris Bushman
WB6EEQ
Colfax, California

---------- FOLLOW-UP ----------

QUESTION: Thanks Chris, I just learned something.

However, I do have a few questions:
When you say a CB antenna without coils .... is that the spring loaded feature that I see on some antennas, or is that some type of feature which improves performance?

I check on the antenna switch ($75) and found the cost to be a little much for my wallet.  I checked on other antenna switches and they were much less expensive ($6 - $20).  It is hard to believe that there is much to a antenna switch that costs so much.  Anyway, is there a specific specification that I should be looking in this antenna switch?

Let us say that I have a 7' foot metal pole at ground level and I install a 2' antenna on the top for a toal of 9' which should work well for 29Mhz.  I then decide that I would like to be capable of operating at 7Mhz but still want to be capable of working in the 29Mhz range.  Which do I do: add 31' of metal pole, add 31' of antenna or mount the old 2' antenna in an offset to the pole while extending the pole by 29' plus installing another 2' antenna?

Thanks.

Jim Kempf

Answer
The "without coils" comment was a cost cutting measure. Sometimes an antenna can be physically shortened by fooling it into thinking it is longer by adding a "coil". This is not the spring thing at the base of some mobile antennas. Since you are using this mobile antenna on your shed you don't need to make it shorter for convenience.

Go with the cheaper antenna switches. The one I pointed out was just an example to get you started.

Now that I look at it, I see that I screwed up in the last message. CB is 27Mhz, not 29Mhz. Sorry.

Take your 7' pole and mount a CB antenna to the top of it. You will only be transmitting on CB freqs so that's where a particular size is critical. Use this antenna for all your radios. You will be attaching the antenna feed line to the antenna at the base of the antenna up at the 7 foot level. The pole is to merely elevate the antenna a bit above the surrounding obstructions.

Chris Bushman