Modified Antenna Whip For Base Antenna To Fit In Attic

[Brillian]

My girlfriend wants to install a base antenna in her attic because she has the same passion as I do for scanning. It's the only option she has because she lives in a town house with no other options. The antenna she purchased is a Antennacraft ST-4 which is the same as the Radio Shack 20-043. We found that the antenna was a bit too long to fit in her attic space.

We decided not to cut down the original stainless steel whip because we don't want to trash a new antenna , but to make a modified version out of copper wire stock which is a close size. This is a picture of what I came up with.

The new copper whip antenna is the same length as the original whip, but is coiled up to fit the attic space.

What do you guys think? Will this coiled up whip work? I would appreciate any comments or thoughts on this modified whip. Is copper okey to use or should we go with stainless steel rod..

Thanks guys ..

 

[prcguy]

The top whip is already base loaded with a coil to resonate around 50MHz. A better approach would be to replace the whip with a shorter one and add a "capacity hat" to the top of the whip.

If you cut the whip length in half I would estimate a metal disk around 6" to 8" in diameter attached to the top of the new whip would resonate around the original 50MHz frequency. If you made the disk as large as 12" diameter the whip would probably resonate lower in the 40MHz region.

The base loading coil for the whip acts like an RF choke (to an extent) at higher frequencies and helps to keep the whip from interacting with the Discone portion of the antenna and degrading performance.

I might solder a pie tin to the end of a 1/8" brazing rod cut to about a 12" and call it a day.
prcguy

 

[Brillian]

We are only interested in the 400 Mhz to 500 MHZ band and 800 Mhz Smartnet using a digital scanner. In this case, is the whip an option or can we get rid of it all together, and still have solid performance.....

 

[prcguy]

The Discone portion of your antenna covers about 100 to 800MHz and the whip is only to raise performance on VHF lo band. Simply unscrew it and that will fix your problem.
prcguy

 

[sleebusjones]

add a "capacity hat" to the top of the whip.

That's a capacitance hat, not a capacity hat.

This would be good reading: MWA Antenna Handbook: chapter 2

 

[rvictor]

Unless access to the attic is difficult, this sounds like a situation where you can try different approaches and see what works best for you. That's the nice thing about a receive antenna--you aren't going to hurt anything by trying. Try to use single site systems for the comparison so you have some basis for the comparison.

Dick

 

[Brillian]

I appreciate all the input. I am going to take your advice prcguy, and build a hat. You are ahead of the curve with antennas. As you can see, I didn't get much done today.

Rvictor, I'm going to mess around with the antenna whip modifications and use a CB radio and a SWR meter to gauge my results before I mount it.

Thanks Guys...

 

[Mike_G_D]

Brillian,

A CB and CB specific SWR meter will only show results that are valid for the CB band (around 27MHz). Unless you intend to transmit through that antenna with the CB I see no reason to do this at all.

As prcguy said, if you are not interested in the low VHF range (30MHz to 50MHz), just leave the whip off altogether. As a true discone this antenna should be fine between about 100MHz and 800MHz or more. The whip is merely there to make it function as a sort of truncated ground plane antenna at the lower frequencies (the "ground radials" being the heavily truncated part). Don't even bother putting a shorter whip with capacitive top hat on it unless you really need to listen to the lower (50MHz) band.

-Mike

 

[kb2vxa]

"That's a capacitance hat, not a capacity hat."
I've never heard a broadcast engineer call it anything other than a capacity hat. Could it be they're not so pedantic? Uh, maybe you could call it a condenser hat just to confuse the new kids on the block. (;->) Anyway the problem has been solved so move along, there's nothing to be seen here.

 

[sleebusjones]

*** deleted, you're not worth the effort ***