An Indoor Antenna Setup

[Sarasota Slim]

I came across the following setup, a while back:

http://statii-radio.blogspot.com/2012/07/how-to-use-cb-auto-antenna-indoor.html?m=1

When I found it, I had just received a Zetagi MM27 (antenna matcher) that I had ordered, and since the thing had come with no instructions for use, I was trying to figure out how to use it. (I've since figured that out, and am pleased with the device )

I notice that the author put a coil in his coax. He includes a picture, and makes mentioned of it. What is the purpose of the coil? Is there a benefit to it, or is it more detrimental to the overall functionality of the system?

I also note, towards the end, that he swaps out antennas, in favor of one that he made; on this antenna, the Zetagi is right there at the base of the antenna. Is there a benefit to moving it closer, like that? He also mentioned that his homebrew antenna needs no ground plane, but I question that.

 

[Robb]

An indoor CB antenna is a poor idea since there are so many factors at work against a good or even fair outcome. Especially when the article's method you brought up isn't even cost effective. I have no doubt this guy did all of this and tells you it works; but you can also bet that it has been so compromised as to not being worth what he invested to get such a small return.

Inside a building, you have both metal pipes and electrical wiring that will mess up the transmit signal radiation pattern. The pipes and wires proximity to the antenna will capacitively detune the antenna itself. Even the aluminum window panes will adversely affect it further in the same manner. Not to mention, I'll just bet his neighbors aren't sharing his same excitement and enthusiasm for CB radio while he is bleeding on to their sound systems and computer speakers.

That article shows a mag mount antenna placed on a small metal grating. If it were bigger in size; that would have been a different story. But that would also be impractical; can you imagine a 5ft x 5ft section of sheet metal in your living room, kitchen, bathroom, or bedroom to give that mag mount sufficient surface to work properly? Not likely. That antenna tuner is noting more than a band-aid that does little except to satisfy his need for a proper SWR. Didn't help that antenna radiate any better by even a single iota.

One could make a horizontal dipole - for almost no money spent at all - and still have the same problems and lack of efficiency. A dipole could be made out of an old 10ft extension cord and a 16ft piece of coax and do a far better job than the proposed mag mount would be able to do; but it would still suffer from all of all the aforementioned negative factors.

Some have been fairly successful using the attic space for antenna. That area has far less problems/inhibitors and seems to work out far better - if this is what you are considering to do. No attic? No bueno. But if you must do this; then save your scratch and make a dipole - IMO . . .

 

[222DBFL]

Yes it can be done. Is it worth it, not if you have a radio in your vehicle!!! Range is very short and it's hard to get a decent match. Been there and done that. Metal AC unit in a hotel room. 2nd floor. Could talk a couple miles if that. And only with an AM radio.
I found it was just easier to go sit in the work van and talk. Then I could shoot DX as well!!
But like said, it's just not a very good way to get on air. Emergency use only and the others your talking to better be close!!

 

[M0GVZ]

I notice that the author put a coil in his coax. He includes a picture, and makes mentioned of it. What is the purpose of the coil? Is there a benefit to it, or is it more detrimental to the overall functionality of the system?

It is being used as a "dirty balun". The idea is to choke off common mode RF on the coax caused by having a poor or non-existent ground plane probably done in his case because every time he keyed up he got a "shock" off the mic or radio. The problem is that the coil needs to be a specific size, diameter and number of turns for the frequency you want it to work on which is the part that most people either overlook or are completely unaware of and think that just making a coil does the job. In his installation its doing the sum total of nothing and may as well not exist.

His ending comment on that page about using a trans match and not needing a ground is wrong. The antenna system needs a ground but his understanding of how antennas work is so lacking that he doesn't know that the coax is actually being used as one.

 

[Sarasota Slim]

Oh, yeah, I have no need or desire to duplicate that guy's system. My house sits on multiple acres, that I own. My wife might complain about antennas, but that's about it. Actually, she'd complain more if I tried to do what that guy did, lol.

Mainly, I was just curious about the coil in the coax. Also, it seemed that I might learn something in discussing this setup, so I stuck it up here. Plus, I figured that if it was truly awful, you guys might get a good laugh out of it.

 

[Sarasota Slim]

In regard to the "dirty balun":

http://www.hamuniverse.com/balun.html

Although, in the above link, it seems to be referred to as "ugly balun".

If the above link is correct, dipoles like baluns; that's good to know, as I utilize a vertical dipole that I assembled from 2 existing 1/4 wave antennas. I suppose that I should look into baluns, then. The above design is a little too large to be practical for my truck, though.

 

[The DB]

Their is both good and bad info on that page. In the first paragraph, for example, they tell us that it is not actually a balun, but a choke, even though it is often called an "ugly balun". However, if you look at their construction instructions, they say that the dimensions are not important as long as you use a certain amount of coax, this is not true, and in fact the opposite of reality. In a circuit like this the dimensions are everything, and the amount of coax used is what is actually irrelevant. I also don't know of one air choke design that doesn't use those so called "expensive" ferrites that can get anywhere close to the bandwidth they are talking about.

Here is a link to more reliable data on the subject. I suggest you especially read the part about why reactive chokes are undesireable.

Also, as a note, I calculated the amount of coax used for the various different air choke dimensions from the chart on the linked page, and only one of them fit in the coax length range that hamuniverse said to use. The only choke that meets the numbers the hamuniverse puts out is the five turns around a seven inch form, and the measured results clearly don't cover the entire HF spectrum link hamuniverse claims.

If you want one choke that will cover the entire HF spectrum their is exactly one to be sure, ferrites.


The DB