Maldol AL-500H VHF/UHF airband whip

[sholt]

I wasn't able to get on the roof tonight, however I was able to toggle between the coil being bypassed this evening during the same transmission, and the noise is less (but still can be overwhelming) when the coil is bypassed.

I also got a chance to try out the attenuator in the same fashion (with bypassed coil). On already relatively clean signals, there was no difference. On middling quality voice, things got better.

I then left the global attenuator on for a while with the psr310 left scanning and it seemed that while I was getting improved results in the middle, I was still getting a fair number of blasts of unintelligible noise through the squelch. Is this related to the possible overload you refer to?

[hertzian]

Ouch. Kind of back to square one with possible overload and weak signal conditions.

You may also want to use the 310's signal-stalker to see if locks up on something consistently. Try using it with the oem duck, tightly squelched and also running normally with the whip. While the results may not be conclusive, it might point in the right direction.

Originally Posted by sholt

I then left the global attenuator on for a while with the psr310 left scanning and it seemed that while I was getting improved results in the middle, I was still getting a fair number of blasts of unintelligible noise through the squelch. Is this related to the possible overload you refer to?

If somebody sees something I'm missing please jump in. At some point, you may end up having to experiment and risk it. Something like the AOR or PAR airband bandpass filters would be good to try. Then, you may still have weak signals to contend with, so an improved antenna up on the deck might be called for.

Remember that everything I've said is a best-effort kind of deal, and you may end up wasting money if the problem isn't solved. But having a bandpass filter that wasn't really needed is good to have for future use.

[sholt]

Hiya,

I have had the OEM antenna on, signal sweeper on with a tight squelch.

I am getting a lot of what seems to be fm signal (I hear music) poking through between 108mhz and 110.5mhz.

I only monitor between 118 and 136 usually so I'm not sure if that could make a difference to my usual monitoring. If not, I think I'll just try and experiment. It wouldn't be terrible to get a filter or an air blade that I can keep in the closet when not in use. Just have to plan for it. Thanks for helping me not make any impulse purchases and I have learned a lot!

[hertzian]

One inexpensive thing you could try is the Radio Shack FM trap/notch filter #15-024 with a hodge-podge of adapters. While this works for many, the notch is high up in the FM band, and the notch depth isn't very deep. BUT, if it makes an improvement, you may want to consider the higher quality AOR or PAR filters. In either case, it would be good to have if you upgrade to a better antenna.

Meanwhile, you can listen to things like KSFO, KOAK etc via
Listen to Live ATC (Air Traffic Control) Communications | LiveATC.net

And lastly, in my experience GRE front ends don't hold up as well in harsh RF environments like Uniden's do. Neither is bulletproof, but is something to consider after all other avenues have been exhausted - no guarantees though.

[DaveIN]

Great information. Just to add, there was also a Comet AL-500H (and S version). Same antenna, different logo, but I believe it is discontinued as well. I have the Pro-43, BC125AT, and an iCOM-R5 that shine on these antennas over the RH77CA.

[RadioDad]

For what it is worth I have a RH77CA, W-889 and W-901L (long version) that I use on my IC-R5.

When I tune to an ATIS signal at the local airport that is on the fringe for me and open the squelch, the best signal comes from the W-901L. The RH77CA and W-899 are close to each other with a slight edge to the W-899.

Any of the three are a big step up from the stock antenna.

[hertzian]

Anything is a step up from the oem antennas.

What made the Maldol so great on vhf airband was that it was NOT a base-loaded or helically-wound whip. It was a center-loaded whip with a coil, which has two great attributes - it is more efficient then the former, AND has more reactance out of band / narrower bandwidth which is good for wide-open scanner front-ends.

A telescopic extended to about 21 inches or so beats them all, yet without the high reactance of a center-loaded coil, may be broad enough that it aggravates the wide-open nature of many scanners if all you want to do is monitor airband.

Note that just having a center loaded coil without the proper element lengths for the band in use does not automatically make it a good airband antenna. Case in point - the RS telescopic when fully extended is actually a coil loaded whip for 45 mhz or so, and acts as a 3/4 wave on airband with a funky response. In that case, bypassing the coil altogether by shorting it by pushing the element just above the coil down about a 1/4 inch until you start to feel resistance results in a better performing standard 1/4 wave for airband if the overall length is still about 21 inches. The RS antenna has it's own thread however.

Is that W-899 only 31cm / 12 inches long as shown in the literature? If so, you'd want a telescopic to be longer than that for vhf airband.

[eorange]

Originally Posted by hertzian

It was a center-loaded whip with a coil

Was this ever confirmed it's really a coil, as opposed to a dielectric junction?

(And no, I'm not cutting mine apart.)

[hertzian]

I did the next best thing - I cut a triband Maldol MH-610 2m/220/70cm antenna apart and found a coil inside. The antenna model is hard to find, and doesn't seem to be in current production - yet is uses the very same materials and center coil. Aside from being unlabeled and a bit shorter in overall length, it could easily be mistaken for the airband unit - and when I bought it I was interested to see how it worked on airband. Result - poorly as compared to the 500H.

My thought here was that as a tribander, it *might* be a usable compromise for civil and milair (2m / 220 coverage), but was inferior to the comet 320A compromise.

What actually happened is when the dikes took the final bite out of the plastic coil covering, the whip flexed and threw some sort of object into the living-room carpet. Drats - oh well, my feet will surely find it later.

However when I look inside the plastic covering, I clearly see coil threads along the entire length in the plastic, so I'll have to assume this is a coil until confirmed by my feet. Thanks - but I wanted to know too.

[hertzian]

Thanks for making me cut the Maldol open to reveal a coil inside. This leads to a better understanding of what's going on.

To cut to the chase, the AL-500H provides the most benefit to low-end scanners with inferior front-ends, while it may not make an eye-popping difference to higher-quality handhelds as compared to a standard 1/4 whip.

Propeller-head analysis:

The primary function of the coil is to decouple the upper half of the whip while monitoring uhf milair. On vhf airband, the coil merely presents a small amount of inductance which shortens the overall length. Maldol engineered it to be resonant precisely in the middle of both bands. The introduction of the decoupling coil makes the LC circuit higher-q than a standard whip. Having ultra-thin elements also help in this regard, almost to the detriment of milair which needs wide elements to cover the extreme bandwidth, but a 2-inch diameter whip is not practical on a handheld for milair.

The MH-610 tribander works the same way - the decoupling coil cuts off the top half of the whip for 220mhz amateur use, but on vhf, the coil shortens the overall length so that the 2m amateur whip also serves as a 3/4 whip on the 70cm band. To do this right, element lengths and coil inductance are tricky, and I felt that the Diamond 320A did this trick a little bit better when measured on an antenna analyzer. Being hi-q, it was not a great substitute for airband monitoring and on 70cm the directional pattern of a 3/4 wave is not usually desired, but directionality may not be so critical for portable handheld.

Back to hi-q resonant AL-500H. The main benefit is that it provides a level of bandpass filtering that is usually not present in low-end scanners. While it doesn't replace proper internal filtering, it does help in areas that are susceptible to FM broadcast or NOAA overload for example. It is also helpful for dual-conversion scanners, especially those that use a 10.7mhz first IF, and being resonant certainly helps especially for low-sensitivity models, yet it won't stop the flamethrower out of band signals from getting through, nor replace a proper external bandpass filter.

Triple-conversion is no guarantee either if the manufacturer doesn't use adequate bandpass filtering. For units that have FM broadcast coverage, a way to reduce costs is to use only a single bandpass filter that covers from 88 - 138 mhz, rather than split these bands up separately with two bandpass filters. Worse yet, a single bandpass filter that covers the entire vhf band from 88 to 175 mhz. Yikes!

In the end, while the AL-500H is admirable, it is not necessarily needed on higher-quality receivers, although it is good engineering practice.

Compared to a standard 1/4 whip, my dual-conversion low-end RS Pro-404 loves it. On my Uniden 396XT, it doesn't seem absolutely necessary.