Modifying a Radio Shack 20-176 ground plane

[W4KRR]

This antenna has a vertical element about 18 inches long for VHF, and it has two 6 inch elements coming off the base of the longer element in a "V" shape. I am wondering if anyone has cut down one (or both?) of the 6 inch elements to 3 inches to attempt to improve reception in the 800 MHz band. If so, did it actually improve 800 MHz reception? Would this degrade the reception in the UHF band? Could you just weld a couple of 3 inch steel rods to the base in the same fashion as the existing UHF elements and expect 800 MHz reception to improve? Is there a limit to how many elements you could add for various bands without degradiog reception on the other bands, or affecting impedance, etc?

 

[tonsoffun]

Hey Ken,
Someone on the boards has done a mod for this but I can't find it!. I remember that they had good results for 800mhz.
I will keep looking
Take care

 

[prcguy]

You could try cutting only one of the UHF elements to 3” and see if 800 improves. Adding a bunch of elements for different bands is not as easy as it seems. Antenna elements that are 1/4, 1/2, 5/8 and maybe up to .64 wavelengths long have a low angle of radiation in a ground plane configuration and this is what you want. At 3/4 wavelength and longer the pattern starts to get multiple lobes and there will be a null at the horizon which will degrade what you want to hear. With that said, elements that are half wave multiples of 1/4 wavelength (1/4, 3/4, 1 1/4, etc) usually have a low enough impedance to transfer energy efficiently in a 50 or 75 ohm system. Half wave multiples of 1/2 wavelength are very high impedance and lossy when connected to 50 ohm coax. So, when you have a 6” 460MHz element and a 3” 800MHz element connected together, the 6” element is 1/2 wavelength long at 800MHz and most of the current will flow to the 3” element and radiate reasonably well. At 460MHz the 3” element is only a pimple and has little effect. If you introduce an 18” 150MHz element it will be 3/4 wavelength long at 460MHz with a low enough impedance to share current with the 6” 460MHz element and waste energy away from the horizon. At 800MHz its also a half wave multiple of 1/4 wavelength (1 1/4) with a low enough impedance to share current and distort the 800MHz pattern. Hams use simple multiple connected dipoles at HF with great success but these are horizontal where the distorted patterns can be useful. A good multi-band vertical can be designed but complex matching and choking would be needed to optimize things and this is where the antenna engineer earns his pay.
prcguy

 

[W4KRR]

IOW, I could add more elements, but they would affect each other, possibly with undesired effects. I guess cutting down one of the UHF elements to 3 inches would be the easiest thing to do.

 

[timmer]

Along these same lines, is there any way I could modify my discone antenna to get better reception on low band? If I could find a longer top element and replace the one that came with my antenna would that make much of a noticeable diffrence?

 

[jonny290]

discones are heavy "compromise antennas" but you can give it a shot by putting a 1/4 wave whip on top. Should get you in the ballpark.

 

[smp928s]

Sorry to revive an ANCIENT thread but I didn't think I should start yet another new one.

I'm wondering if there is an easy way to modify this antenna so that it receives just a little better in the 154-156mhz range. Basically there are several conventional frequencies in this range that I would like to monitor that I can often receive but just not enough to comfortably monitor. I'm limited to keeping the antenna indoors. I have a very short cable run and have tried various cables, including a short run of LMR-400 and tried moving the antenna to various locations in the apartment. I believe the top elements are permanent but can obviously be trimmed. I know that there are 800mhz mods to the UHF elements. My knowledge of the science behind antennas is basically nil but I'm trying to learn. Sacrificing other bands is a chance I'm willing to take if it will improve the 154-156mhz performance.

Thanks!

 

[trp2525]

...I'm wondering if there is an easy way to modify this antenna so that it receives just a little better in the 154-156mhz range. Basically there are several conventional frequencies in this range that I would like to monitor that I can often receive but just not enough to comfortably monitor...Sacrificing other bands is a chance I'm willing to take if it will improve the 154-156mhz performance...

You might want to try this 1/4 wave VHF-High ground plane antenna from Centerfire Antenna. They offer 4 different models that cover different frequency ranges in the VHF bands (108-118, 118-136, 136-155 and 150-165) and the one that most closely covers the 154-156 MHz range that you are looking for is the 150-165 MHz model (Weather, Marine and Railroad Band) for $29.95 (plus $8 Priority Shipping): VHF Special Purpose – Centerfire Antenna Mfg Co

 

[mmckenna]

Sorry to revive an ANCIENT thread but I didn't think I should start yet another new one.

I'm wondering if there is an easy way to modify this antenna so that it receives just a little better in the 154-156mhz range. Basically there are several conventional frequencies in this range that I would like to monitor that I can often receive but just not enough to comfortably monitor. I'm limited to keeping the antenna indoors. I have a very short cable run and have tried various cables, including a short run of LMR-400 and tried moving the antenna to various locations in the apartment. I believe the top elements are permanent but can obviously be trimmed. I know that there are 800mhz mods to the UHF elements. My knowledge of the science behind antennas is basically nil but I'm trying to learn. Sacrificing other bands is a chance I'm willing to take if it will improve the 154-156mhz performance.

Thanks!

The antenna is a 1/4 wave ground plane at VHF with a pair of 1/4 wave elements added to help with UHF reception.

Replacing one 1/4 wave antenna with another isn't likely to make any difference.
Same with trimming it for exactly 1/4 wavelength at 154-156MHz. For receiving, it just isn't that critical.

Upgrading coaxial cable is often an easy way to squeeze a bit of performance out of an antenna system, but in your case, the minimal length likely isn't going to make enough difference to notice.

Indoors it's kind of hard to fit a "gain" antenna in there. You could try, but a 5/8ths wave antenna is going to be about 47 inches long, and it'll still require the ground plane. Might be worth trying a 5/8th's wave NMO mount mobile antenna with the base station adapter kit. It's not going to be cheap and is only going to give you 3dB more gain, which may or may not make enough difference for you.

Of course you realize the core issue here is that you need to keep the antenna inside. That creates issues with shielding from building materials and local noise sources,
You could try a preamplifier, but I suspect that isn't going to help much. While it can increase signal strength, it'll also increase the interference from local noise sources.

Is there any way at all to get a stealth antenna outdoors?

 

[trp2525]

...Indoors it's kind of hard to fit a "gain" antenna in there. You could try, but a 5/8ths wave antenna is going to be about 47 inches long, and it'll still require the ground plane. Might be worth trying a 5/8th's wave NMO mount mobile antenna with the base station adapter kit. It's not going to be cheap and is only going to give you 3dB more gain, which may or may not make enough difference for you...

Antenna Specialists (when they were in business) manufactured a MONR17 base station ground plane monitor antenna that was a "gain" antenna and specifically covered 130-174 MHz according to its packaging (see attached pictures). I can tell you that the length of each of the 4 ground-plane radials of this antenna is 17 inches and the length of the center vertical element looks to be in the ballpark of 47 inches which is similar to what RR member mmckenna mentioned above regarding a 5/8 wave VHF-High gain antenna. Keep an eye out on eBay and other radio/scanner classified sites as I have seen this A/S MONR17 model pop up from time-to-time.

 

[br0adband]

Holy thread necromancy, Batman!

Personally I'd say grab a few wire coat hangers, an SO-239 chassis mount, some spade lugs and screws, and make your own 1/4 wave ground plane cut to 155 MHz and be done with it. Can be constructed in 10 minutes if you have the parts, literally, maybe a bit longer straightening out the coat hanger elements. Make the ground radials about 1 inch longer than the main element cut to 155 MHz and voila, home brew and it will perform incredibly well and save that RS antenna from being mauled in whatever process you were hoping to do.

Sure you can buy a 1/4 wave ground plane for that frequency/purpose but honestly make one yourself and be done with it, that's my advice. These are the most basic plans for such an antenna and they're just incredibly easy to make in a short amount of time with minimal cost:

2 meter 1/4 wave Ground Plane

For 155 MHz you'd be looking at 18.1 inches long for the main vertical element (A in that diagram) and say 19 inches for the ground plane elements (B&C/D&E) and you're good to go. I've made dozens of these for friends and I have two of them myself that I use pretty regularly - one good thing because mine are cut to 150 MHz is they work fantastic on the harmonics at 450 and 900 (and it covers the 700-800 MHz range just fine too).

 

[smp928s]

Thanks for the responses. I might try making an antenna but I think I might go the route of the NMO antenna on a base ground plane kit and give that a try.

 

[trp2525]

...I think I might go the route of the NMO antenna on a base ground plane kit and give that a try.

You might want to consider this MBCN base station ground-plane kit from Laird which terminates in an "N" connector instead of the typical SO-239 connector that mates with a PL-259. While the "N" connector won't really make much of a difference as far as loss goes in the VHF-High band, if you choose to use the same ground-plane kit with a UHF or 700-800-900 MHz NMO antenna in the future, it would be the better choice.

Link to Laird MBCN base station ground-plane kit for NMO antennas: http://www.theantennafarm.com/catalog/laird-technologies-mbcn-7531.html

 

[smp928s]

Thanks for that suggestion. I believe that is the route I'm going to try.

I have an 800mhz yagi connected to a dedicated scanner for the one 800mhz trunked system I like to monitor. It makes it so that I can hear that system about 75% of the time with a little finessing.

Being in an apartment in Upstate NY (mobile scanning is illegal) with no way to get an antenna outside and at an elevation of about 250' is a strain on the hobby!

I've actually been looking at houses in areas of the highest elevation around me (1500'+) haha. When I was a teenager I had an antenna about 30' up off my parent's barn and it spoiled me.

 

[N9JIG]

What would likely be more effective then adding to or otherwise modifying the antenna would likely be raising it a few yards. Adding 10 or 20 feet in elevation would increase the distance from where you can receive.

While this might not be practical all the time it will work better than changing the elements of the antenna.

 

[smp928s]

I wish I could Rich. There is just nowhere for me to raise it to. I can't get it outside and I literally have the top element touching the ceiling haha.

 

[cbehr91]

Sorry to revive an ANCIENT thread but I didn't think I should start yet another new one.

I'm wondering if there is an easy way to modify this antenna so that it receives just a little better in the 154-156mhz range. Basically there are several conventional frequencies in this range that I would like to monitor that I can often receive but just not enough to comfortably monitor. I'm limited to keeping the antenna indoors. I have a very short cable run and have tried various cables, including a short run of LMR-400 and tried moving the antenna to various locations in the apartment. I believe the top elements are permanent but can obviously be trimmed. I know that there are 800mhz mods to the UHF elements. My knowledge of the science behind antennas is basically nil but I'm trying to learn. Sacrificing other bands is a chance I'm willing to take if it will improve the 154-156mhz performance.

Thanks!

One of the tricks for making these antennas a 1/4 wave on the rail band (~161 mhz) was to bend the ground radials down to a 45 degree angle, removing the two shorter elements and removing and cutting the main vertical element to frequency, which in your case would be a tad over 18".

 

[scanmanmi]

Try building one

I made one out of copper wire on pvc just to see what it was like and it was amazing although I didn't have any way of testing it. It was just an experiment and couldn't be mounted on a tower which was the plan but may work in an apartment. For the 5/8 part of it you have to make some windings. Something like this: 5-8 Ground Plane

 

[riccom]

i did a mod where i took all the top elements off, then added a single vhf 18 inch whip only for vhf transmit, and my swr was a 1.5 at 30 watts.
it did very well on the rx sid and tx i hit repeaters at 50 miles away.

 

[prcguy]

There is no reason to bend the ground radials down or remove the shorter elements for 161MHz, just trim the longer whip a little.
prcguy

One of the tricks for making these antennas a 1/4 wave on the rail band (~161 mhz) was to bend the ground radials down to a 45 degree angle, removing the two shorter elements and removing and cutting the main vertical element to frequency, which in your case would be a tad over 18".