Fan Dipole Cutting Questions

[LIScanner101]

I have a Channel Master Monitenna 5094A that I am trying to put in my attic due to the wicked winds we have gotten here on Long Island last couple years. It's about 10' tall so I know there's no way it's going to fit vertically. I am almost ready to put it up there and just tuck the bottom element down inside a wall (there'd be sheetrock on both sides) but this will leave several feet covered by something solid. I know this is going to affect the Rx capability but I think tilting the antenna sideways will be almost as bad - almost like a "six of these, half a dozen of the other" situations.

Here is the frequency coverage of this antenna:

Low band 30-50 Mhz
High band 148-175Mhz
UHF 450-470Mhz
UHF-T 470-512Mhz
UHF-HB 800-912Mhz

Nothing I currently monitor on Long Island is lower than 46MHz. Therefore, I was wondering if I can just cut-down the vertical elements a little to tune the low-end coverage to 46MHz? This will also give me a shorter antenna that would fit better in the attic.

Is there some formula I need to follow in order to determine the proper cut-down length of my Monitenna? I know with whips it's pretty straightforward, but with a fan dipole like the Monitenna it might be more complex...

Thanks,
-John

 

[nanZor]

I wouldn't cut that down:

234 / F mhz = length in feet (quarter wave each side), so at 46 mhz, that's about 5 feet needed on each side. If you cut it, you'll be raising the resonant frequency above 46 mhz.

Then again, placing it against sheetrock will do the same so.... but man, I wouldn't want to cut a legendary antenna like that.

If you were willing to risk destroying a classic, consider cutting down the taller elements and replacing them with a "T" capacity load. That is, if you cut a side down by a foot, attach a TWO foot element (total end to end length) like a "T" right in the middle at the end of the vertical - so one horizontal foot on each side of the vertical.

 

[TeRayCodA]

Reinforce!

Why don't you do the "Bicycle fiberglass flag-pole" modification to this antenna,instead of compromising it's intended design?The instructions are under the WIKI.I have had a Scantenna,and Channelmaster Monitenna like yours,up for years,with no damage at all. A "T" capacity load?? I don't quite understand...Just do it this way,and mount it the way it was intended.You'll be happy with it.I live in Tornado Alley.It works!!!

 

[TeRayCodA]

http://wiki.radioreference.com/images/8/8c/Reinforcescantenna.jpg

I should know,I submitted this a few years ago.Had trouble finding it.

 

[LIScanner101]

Thanks all for the great input – I LOVE this site!!!

I’ve known about the bicycle flag-pole mod for a while, but I didn’t know it could be applied to the Monitenna. I have a few Scanntennas still NIB but they are a lot shorter than the Channelmaster gem. I guess I can try that. However, the OTHER reason I haven’t mounted it outside is due to aesthetics and something else called “the boss” (i.e., the wifey). She’s not going to be too pleased with that thing on the chimney but I’ll “work” on that…!

Now about the length. I understand the 234/F MHz (actually I just use speed of light and convert the resultant miles to feet or inches but either way works). However, I didn’t know that could be applied to a fan dipole. The Monitenna has these funky “horseshoe-shaped” elements that stick out on the shorter elements and I thought those had some part in determining the length. Which now brings me to my NEXT question, which is, if 5’ is resonant at 46MHz then why did Channelmaster claim 30-50MHz coverage? Shouldn’t the antenna elements be around 7.8’ in length then?

 

[LIScanner101]

If you were willing to risk destroying a classic, consider cutting down the taller elements and replacing them with a "T" capacity load. That is, if you cut a side down by a foot, attach a TWO foot element (total end to end length) like a "T" right in the middle at the end of the vertical - so one horizontal foot on each side of the vertical.

OK, so if I cut down the top & bottom by 1' each, I would need to add a HORIZONTAL 2' long element on the end of the top & bottom element, correct?

 

[nanZor]

We'd have to put the Channelmaster on an analyzer, along with an electrical half-wave of coax to see where it is resonant. To cover 30-50 mhz, they most likely just put resonance somewhere in the middle near 40mhz or so.

As for the "T": this is known as "end loading". When you shorten an antenna, you can reestablish resonance by putting an inductive coil at the bottom, or the center. As you move the coil further up the element, the coil gets larger and larger and impractically large if placed at the end.

The other option is to use a capacitive reactance at the end, rather than an inductive coil. Usually this is known as a "top hat" and seen as a disk or wagon-wheel of rods. It is top heavy and a wind-load, so unless it is very small it is usually guyed or fixed in place with supports.

In your case inside the attic, with no wind-load, this would be an option. The simplest form of top-hat is just two horizontal wires (or maybe 1/4 inch tubing). Think of it this way - you need to bend over your elements by a foot to make it fit. To make it work like a capacity hat, you need to place the missing horizontal foot on the other side. So a grand total of two feet horizontally, attached to the vertical element in the middle of the horizontal tubing, ie one foot each side.

If you were to use FOUR crossed end-loading wires/rods, they would be about 6 inches in length each. The more end loading rods you have, the shorter they can be individually, but this two-element top-hat works well enough since the lengths are manageable.

When current (either transmit or receive) hits the horizontal wires, it splits in two and are out of phase, thus cancelling each other. All that is left is basically the plate of a capacitor, which establishes resonance by reintroducing the capacitance missing from the original longer vertical.

I use this trick on vhf too - converting amateur 2-meter 144 mhz antennas down to about 128 mhz by putting a small top-hat on it if I just can't extend the vertical element itself. Two 4-inch wires at the end and a hose-clamp gets me in the ballpark for temporary duty.

 

[LIScanner101]

We'd have to put the Channelmaster on an analyzer, along with an electrical half-wave of coax to see where it is resonant. To cover 30-50 mhz, they most likely just put resonance somewhere in the middle near 40mhz or so.

Interesting stuff….
I’ve never bothered checking an antenna’s resonance because I don’t have an analyzer and for what I do (just scanning) it’s probably not going to pay for itself over the course of my lifetime but I do see an MJF pop-up add on this site, maybe I’ll check it out. Also, I was unaware that you need to connect a half-wave of coax between the antenna and the analyzer. But how would you know what length of wire to use if you're trying to determine the antenna's resonance? Isn't that like putting the cart before the horse? I’d be trying to determine what frequency the Monitenna is resonant at – isn’t cutting a half-wave of coax assuming that I would already know what it is, and just verifying it? Or would I need to actually start with the longest piece (say, cut for 30MHz), test the antenna and then cut the coax a little at a time until I find resonance? Maybe I misunderstood you…

The other option is to use a capacitive reactance at the end, rather than an inductive coil. Usually this is known as a "top hat" and seen as a disk or wagon-wheel of rods. It is top heavy and a wind-load, so unless it is very small it is usually guyed or fixed in place with supports. In your case inside the attic, with no wind-load, this would be an option. The simplest form of top-hat is just two horizontal wires (or maybe 1/4 inch tubing). Think of it this way - you need to bend over your elements by a foot to make it fit. To make it work like a capacity hat, you need to place the missing horizontal foot on the other side. So a grand total of two feet horizontally, attached to the vertical element in the middle of the horizontal tubing, ie one foot each side. If you were to use FOUR crossed end-loading wires/rods, they would be about 6 inches in length each. The more end loading rods you have, the shorter they can be individually, but this two-element top-hat works well enough since the lengths are manageable.

Cool, so suppose I could get away with cutting-down the top & bottom elements 1’ each AND I was able to get my hands on a metal disk for each end. Would it be BAD to make these disks 2’ in diameter (assuming I could) and attach them in the center to the ends of each long vertical element? Or does each disk HAVE to be smaller in diameter? I’d like to go with the best Rx option – if disks work better than x number of radial wires I’m OK with that.

 

[TeRayCodA]

Why do you want to cut on this antenna? It works great,right out of the box. You don't really need to 'sweep' this antenna,It's not for transmitting.Also,it's 300 ohm to 75 ohm.Maybe I 'm missing something here?

 

[TeRayCodA]

The Monitenna is a Dipole "Cluster design" it's made to receive a very broad band.I would imagine sweeping it,would yield interesting results,but,it's actually a 300-75ohm antenna.I don't know what this mis-match would look like on a MFJ SWR/Antenna analyzer.Mine works fine the way it is. They(Channelmaster) did their 'homework' at the lab,All i can say is;It works great w/out any modification.I've had one up for years.

 

[TeRayCodA]

The longest element on the 5094A,is the VHF-lo band receive.Yes,you could probably get away with cutting it down,if low band is of no interest to you.Don't bother the other elements.You would like it soooo much better mounted on that chimney! but,I understand.Like Hertzian said,you can cheat,a little with the 'top hat' Idea.

 

[LIScanner101]

Why do you want to cut on this antenna? It works great,right out of the box. You don't really need to 'sweep' this antenna,It's not for transmitting.Also,it's 300 ohm to 75 ohm.Maybe I 'm missing something here?

Sorry, I don't WANT to cut this antenna, please see my OP. I feel that I HAVE to because it's most likely not going outside and it's not going to fit into my attic uncut.

 

[LIScanner101]

The longest element on the 5094A,is the VHF-lo band receive.Yes,you could probably get away with cutting it down,if low band is of no interest to you.Don't bother the other elements.You would like it soooo much better mounted on that chimney! but,I understand.Like Hertzian said,you can cheat,a little with the 'top hat' Idea.

As already said, I don't WANT to cut the antenna, but if I can't install it outside I don't think I have much choice...I agree, it would suck to cut it but I may have to...

Getting back to the length, I would still love to know what frequency the longest elements are resonant at. If they are resonant in the mid-40's then I understand that I'll have to do the "top hat" trick if I cut it, but if it's resonant at a LOWER frequency then I should be able to cut it until it is resonant at a higher frequency - would be an interesting experiment...

 

[nanZor]

I think we're getting way ahead of ourselves here. Having an analyzer gives me an indication of the general resonance, but being very super precise about it a few mhz one way or the other is not an issue for rx-only applications like scanning.

It is a tool that can easily be abused by chasing low swr's when that is not the only thing that determines antenna efficiency.

Yes, if you were to make a solid disk instead of the two end-loaded wires, the disk would be much smaller - more surface means more capacitance.

BUT, our task is not to change the original resonance. With only two horizontal wires, the original resonance is very close when you merely put the same "missing length" of a shortened vertical on each side in a T-fashion. It is the easiest way to go. (You don't have to make them actually two separate wires - it can be a solid rod just attached to the middle.)

And putting in a big CYA statement - I have never done this on a channelmaster, although I doubt that anything else is relying on the low-band elements for double-duty on other freqs.