? for the antenna experts

[dave3825]

I have a 20 176 Rat Shack sputnik and have read its peak performance is in the 152-470MHz range. Its a dual band ant. It has a long radial for vhf and 2 shorter ones for uhf. Have not mounted it yet.

I saw online a Centerfire tri band and its peak Receive: 150-170 Mhz, 420-470 Mhz, 825-875 Mhz.

The only visual difference that I see is centerfire has 4 radials as the ground plane and Rat Shack has 3. They both have 3 radials on top. Centerfire radials are 3 different sizes. 1 for each band. The Rat shack has 3 also but 1 for vhf and 2 the same size for uhf.

If I shortened one of the uhf radials on the Rat Shack, would that make it a tri band?

And if so, I have 2 distant 500 and 770mhz systems as well as local 700 and 800 systems. I would like to know opinions of cutting one uhf radial for 600mhz. to get decent on 5/7/800 mhz systems.

Thanks

pics and specs
Ratshack Amazon.com: New RadioShack Outdoor VHF-Hi/UHF Scanner Antenna 20-176: Home Audio & Theater

Centerfire VHF-Hi/UHF Multi-Band

 

[chief21]

Theoretically, antennas cut to a specific frequency will work better on that frequency than those that are not. Whether it would make any significant difference is the question. Also, keep in mind that antenna design (and RF in general) is 95% physics and 5% magic, so adjusting parts of an antenna without knowing the design intentions could have undesired effects. On the other hand, part of the fun is experimenting... Right?

 

[jwt873]

The Rad Shack antenna only has three radials. That's the three on the bottom. They make up the ground plane. Same with the Centerfire antenna it has four radials.

The long one and two short ones extending up from the middle of the antennas are the 'driven elements'.. They do the actual receiving. The long one is for VHF the two short ones are UHF. Can't tell on the Rad Shack, but on the Centerfire they are slightly different lengths so so they are resonant at slightly different frequencies.. Cutting one of them down to be resonant in the 800 Mhz band might just work.. (About 3.75 inches) See --> Online Conversion - Frequency Wavelength Calculator

Then again, might not make any difference at all.. But like chief21 says above.. Part of the fun is experimenting.

 

[Arkmood]

No expert, have commented on the 20-176 before... have/had a couple and engaged in modifications/disassembly - with these observations:

Any deviation from the original design resulted in what appeared to be unpredictable results - from the removal of the ground plane(s) to popping off the black cap(s) at the end of radials/gps. All pointed to a change in antenna properties...

Opinion: find fault with the Centerfire design and not surprisingly no real specs are given/page content is protected...

If modification is to take place with 20-176 uhf radials would suggest cut both to the same desired length using legit antenna/freq calculator...

 

[dave3825]

Theoretically, antennas cut to a specific frequency will work better on that frequency than those that are not.

That's why I am looking at maybe something cut for the middle of 500mhz and 773mhz , 636.5mhz to be exact. That would make it about 135mhz lower than 773mhz and 135mhz higher than 500mhz. Where it gets interesting is the uhf of 470mhz is only 30mhz from 500mhz. So it makes me wonder if the driver I want for 500/700mhz should be cut closer to 700/800.

On the other hand, part of the fun is experimenting... Right?

That's the best part. I love to tinker. Just don't like going up on the roof at my age any more than I have too.

Can't tell on the Rad Shack, but on the Centerfire they are slightly different lengths so so they are resonant at slightly different frequencies.

The centerfire is a tri band and they are 3 different lengths, The center is vhf, the two that make up the V , 1 is for uhf and the other is 800mhz.

If modification is to take place with 20-176 uhf radials would suggest cut both to the same desired length using legit antenna/freq calculator...

I had wanted to cut only one down in hopes of mimicking the centerfire, and making the R S dual band, into a tri band.

I have it soaking in oil as the drivers on the RS are threaded on one end so when I figure the thread size, I can get a die. I have many antenna parts from over the years so there's a good chance I have something the same thickness.

 

[iMONITOR]

I have many antenna parts from over the years so there's a good chance I have something the same thickness.

For stainless steel rod check ACE Hardware or Home Depot if you don't have anything.

 

[Arkmood]

I had wanted to cut only one down in hopes of mimicking the centerfire, and making the R S dual band, into a tri band.

I have it soaking in oil as the drivers on the RS are threaded on one end so when I figure the thread size

Yes, comprehend the concept of your project, and that's where my misgivings arise...
Never pictured the receiving element(s) as being "threaded" nor "drivers"/"driven elements" - please let me know how it works out...