Cut antenna - Same performance?

[Patrick_]

I just purchased a Super Scan Duck from Antenna Warehouse. It's the exact same length and style as an RH77CA... just rebranded. It's a good antenna -- much better at all bands over the stock, with the exception of 800MHz, which is _slightly_ worse than the stock, but virtually unnoticeable unless I am on the fringe. It's okay, however, because the 800MHz TRS I listen to is strong enough for me.

I have a few systems I listen to that are still conventional. When they switch to an 800MHz TRS (which they inevitably will), I'll probably want a bit better 800MHz performance than what I have.

So, I've read that the Seeker 800MHz antenna is probably the best for 800MHz scanning, if only a bit better than the RS800MHz. It's length is 11.75". The RS800MHz length is about 7".

So, let's just say I were to cut the 15.75" Scan Duck down to 11.75". Theoretically, could the cut antenna's performance then be at least almost the same as the Seeker?

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I'd also like to make an interesting note: most people here have stated that the Seeker has better 800MHz performance than the RS800MHz. This is interesting, because if the Seeker was a 1/4 length, the resonant frequency would be 238.98MHz. If it were 1/2 length, the resonant frequency would be 477.96MHz.

Calculating the RS800MHz resonant frequency, if it were a 1/2 length, it would be resonant at 802MHz! How could the Seeker be better at 800MHz?

 

[DPD1]

It's not just the length... There's a matching coil in the bottom. At least on the 77 there is. I'm not sure about the other one. But you have to change both the coil and vertical element and balance between them to get the point of resonance. So trimming isn't a good idea usually, unless it's for sure just a basic whip with no coil.

Dave
www.DPDProductions.com
Custom Antennas & Radio Accessories

 

[Patrick_]

Good point. Let's say, however, that I knew the length of the coil inside the antenna. How would it be if I cut accordingly?

Or would the fact that the length is mostly made of coil make the performance suffer?

 

[CWR]

I recently purchased a couple of Maldol MH-610 Tri-band 2M/220/440 ducks. This antenna has a set-screw that holds the whip in the loading coil. Right out of the package it was ok on 800MHz but not great. I got an idea , and backed-out the set screw , pulled the whip out & trimmed about an inch off of the bottom, stuck it back in the loading coil & tighened the set-screw. The antenna came to life on 800MHz. I'm currently running 2-Pro-2096's with MH-610's on sma-female to bnc male adapters & bnc righ-angle connectors.

I once ran a Seeker-800 on a BNC-Clip mount , clipped to the roof of an RV and found it worked well o VHF-Hi, UHF, & 800MHz.

I bought a Diamond RH-951S , that's supposed to RX on 150/300/450/800 & 900 MHz. I couldn't even get it to break squelch on 800MHz.

Also for VHF-Hi, UHF, & 800MHz the Watson W881 Super-Gainer,
and the Watson W-889 Tele-Gainer are hard to beat..........

 

[DPD1]

Good point. Let's say, however, that I knew the length of the coil inside the antenna. How would it be if I cut accordingly?

Or would the fact that the length is mostly made of coil make the performance suffer?

The problem is, unless you have some kind of field monitor or analyzer, it's almost impossible to know what's going on. It's a lot more than just physical dimensions. Many people think that xx inches is supposed to equal a certain frequency in half wave, 1/4 wave, whatever... When in reality, you'd have better luck hitting the lottery than hitting the resonance point just right, simply by guessing with physical dimensions. Lots of the time (actually most of the time) physical dimensions do not do what they should do electrically. It's just a starting point. Then on the other hand, when you're as high as 800, the bandwidth tends to be pretty wide anyway and leaves a lot of room for error. I guess it depends on whether you think it's worth messing with.

Dave
www.DPDProductions.com
Custom Antennas & Radio Accessories

 

[jparks29]

The problem is, unless you have some kind of field monitor or analyzer, it's almost impossible to know what's going on. It's a lot more than just physical dimensions. Many people think that xx inches is supposed to equal a certain frequency in half wave, 1/4 wave, whatever... When in reality, you'd have better luck hitting the lottery than hitting the resonance point just right, simply by guessing with physical dimensions. Lots of the time (actually most of the time) physical dimensions do not do what they should do electrically. It's just a starting point. Then on the other hand, when you're as high as 800, the bandwidth tends to be pretty wide anyway and leaves a lot of room for error. I guess it depends on whether you think it's worth messing with.

Dave
www.DPDProductions.com
Custom Antennas & Radio Accessories

Consider me lucky then.. I had a 5/8ths from maxrad for VHF that I cut for 2m... I was expecting it to be off a bit, damn if my SWR meter didn't register 1:1.... cutting charts work

 

[Patrick_]

Well... I guess when the system moves to 800MHz, I'll start cutting the antenna by a 1/2" until 800MHz performance gets better.

 

[zz0468]

I must be missing something here. Why do you need to cut this antenna? Cutting one make and model of antenna to be the same length as another make and model antenna doesn't necessarily mean a thing.

 

[Patrick_]

Well... for one thing, the Seeker is $40. I bought my Scan Duck for $25, including shipping.

 

[Patrick_]

I emailed Antenna Warehouse out of curiosity, to see if they knew the length of the coil. They responded:

"There is no base loading coil, but rather Two L/C electrical 'trap' coils along it's length that isolate portions of the antenna depending on frequency. These traps do add some loading
inductance on the lower band thus making the 15 inches enough to resonant the antenna in the 150-157 Mhz. band."