Ham antenna and other questions

[AllanD]

I suspect the vast majority of that went completely right over the OP's head lol

But yes, that cavity duplexer is the component that would be difficult to keep aligned with the constant vibration of a mobile environment.

FWIW, radios used in crossband repeat mode typically have an extremely short carrier hang with no courtesy beep (if they have a carrier hang at all, many de-key the xmtr immediately upon the squelch closing, creating the illusion it's not a repeater at all). It's not the same thing as with a typical full-duplex repeater. Where you still can get locked out however is if someone starts jamming the frequency with a continuous signal.

My Icom 2410H doesn't have a courtesy beep, What I was refering to was that if I've got it in repeater mode to contact a repeater (parking the truck on a hill helps) while I'm Kicking around in a valley and trying to reach the the truck and thus the distant repeater via my handheld on 440 simplex if everyone using the repeater keeps the repeater from dropping out after the courtesy beep they will NEVER hear a word I say, unless they LET the carrier drop my dual band mobile radio will not stop repeating "out" to me on 440....

And yes, the IC-2410 drops INSTANTLY

As for "jouncing around" destabilizing a cavity duplexer?

First the fact that it takes up as much space as two 72 quart coleman \collers stacked on top of each other (think of a box big enough that you can fit not one but TWO 31x10.5 tires inside) means you don't want one in your truck anyway.

The fact that there are delicate pieces that simply won't work if moved, like a concert piano that WILL get out of tune by simply moving it across the room. (or even across a smooth stage or concrete floor)

a typical six-pack (or 8pack) duplexer is destabilized/detuned by a change in temp of only a few degrees, so jouncing is the least of your worries.

I helped build and install a 6meter duplexer we constructed the cavities
out of stainless steel that we first etched, then nickel plated then did a heavy copper electro-plate. after we tig welded them together...

you think a 2meter duplexer is twitchy? try a six meter duplexer sometime.
1/2degree F will detune it. if you have in a temp controlled room even opening the door to go in or out knocks it out of tune for 10-15min...

the guy I was building it for originally thought it would be impossible to keep it reasonably stable even in a climate controlled room....

that was before I suggested keeping them in a liquid (Non-Toxic antifreeze)
bath, the temp was regulated by commercial aquarium heaters. and inside the transmitter room of the UHF TV broadcast facility that was providing the tower space... they had nothing else going on at the 200foor level of their 575foot tower

If you want stability thermal mass is your friend

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[Spott]

Huh! Never though of dunking the cavity filters, but in retrospect, it seems like a great idea.

Also, the shape of the cavity can determine a great deal about the tunability. A short, fat space reacts the least to thermal change, but is also the most difficult to tune initially. A long, narrow space tunes most easily, but will swing a lot further off frequency with temperature differences. Around a 3:1 height/width ratio is usually a good compromise. And of course, cylinders are a lot simpler to calculate and construct than boxes.

Spott

 

[4x4junkie]

My Icom 2410H doesn't have a courtesy beep, What I was refering to was that if I've got it in repeater mode to contact a repeater (parking the truck on a hill helps) while I'm Kicking around in a valley and trying to reach the the truck and thus the distant repeater via my handheld on 440 simplex if everyone using the repeater keeps the repeater from dropping out after the courtesy beep they will NEVER hear a word I say, unless they LET the carrier drop my dual band mobile radio will not stop repeating "out" to me on 440....

Good point, I didn't think of it that way, though typically if there's a real repeater available, wouldn't you just use it (or at least listen to it) directly? (I guess unless it's such a deep valley you don't hear it, but that doesn't seem like a real typical case, at least to me anyway)

Years ago I built a VHF cavity duplexer out of 4" type-L copper pipe. Polished the insides (and the center 1.5" dia tuning rod) to a bright reddish-pink luster with fine steel wool, and then coated it with an acrylic clear spray coating to preserve it. It's just inside the living space of my house, but I never really noticed enough change from temperature on it that it affected it's operation The SWR into the TX side fluctuated up to maybe 1.3:1 at the most that I saw (granted, I also was not using it at the very close-in TX/RX spacing of 600kHz either). Moving it out of the room and back again made it jump up to 1.6:1 though, which meant I had to retune it.
I got the design for it (and a "Super J-Pole" antenna which I also constructed from copper pipe) out of the Antenna Book many years ago ('88? I think)

I guess this thread is pretty much completely hi-jacked now huh...? At least it would appear the OP's question has been answered already

 

[--weezl--]

don't mean to interupt your guy's conversation... but you have completely lost me...

that being said, don't worry about the thread jack, it's a relevant conversation, and is both informative and positive, so keep on truckin

Good point, I didn't think of it that way, though typically if there's a real repeater available, wouldn't you just use it (or at least listen to it) directly? (I guess unless it's such a deep valley you don't hear it, but that doesn't seem like a real typical case, at least to me anyway)

to assume you are still talking about cross band repeating or such, it's basically to allow a group of people who have smaller (and lower powered radios, including FRS) to communicate with people who are in the city, while we are in the bush... that being said, the reason they don't just use their vhf or uhf radios to communicate with people in the city, is because the areas we 4x4ing in get poor reception from the vhf repeaters to begin with, and the 2900 i'm looking at has a much stronger (75w) transmitter than the handheld i've got with a 4w transmitter...

 

[Spott]

75w will get the job done. I think most of the repeaters in this area are 50w transmitters, although they have nice tall towers with fancy antennas.

Just make sure you're parked up on top of a hill or something...all the power in the world won't help you if you're trying to transmit through rock and dirt.

If everyone had little UHF handhelds, then a crossband repeater would work out great.

Spott

 

[--weezl--]

really? only 50w? i woulda figured it would be like 100+

i'm looking at the yaesu 2900, which after going over their product line, looks like it's the highest power mobile unit... it's dual band too, which is nice

 

[4x4junkie]

to assume you are still talking about cross band repeating or such, it's basically to allow a group of people who have smaller (and lower powered radios, including FRS) to communicate with people who are in the city, while we are in the bush... that being said, the reason they don't just use their vhf or uhf radios to communicate with people in the city, is because the areas we 4x4ing in get poor reception from the vhf repeaters to begin with, and the 2900 i'm looking at has a much stronger (75w) transmitter than the handheld i've got with a 4w transmitter...

I would agree, with a bunch of little UHF radios around, a cross-band repeater certainly could come in handy. However like AllanD brought up, cross-linking into another repeater (while listening to the repeater through the cross-link) does lock you out until the repeater's carrier drops, so you might be locked out awhile if there's a bunch of ratchet-jaws out there (though you could get lucky... Seems I recall a very small number of repeaters drop their PL tone right as soon as there's no input signal, so setting the tone squelch decoder on your rig might speed up your opportunity to key up into the system if this is the case). Otherwise, if you're able to hear the repeater directly on your handheld (it would need to be dual-band unit), then that would avoid the issue entirely (you'd need to set your crossband radio to not rx the repeater's output though).

 

[AllanD]

really? only 50w? i woulda figured it would be like 100+

i'm looking at the yaesu 2900, which after going over their product line, looks like it's the highest power mobile unit... it's dual band too, which is nice

Consider where mst repeaters are sited, on Water towers or the 200foot level of a commercial FM or TV broadcast tower are real popular.


And one overlooked thing about repeater sites is that the antennae made for high mounted repeater ops are specifically made slightly off the ideal length.
The thing is if you take an antenna that slightly shorter (or was it longer?) than the ideal cut for frequency and you then sharply tun it onto frequency he annular antanna pattern will have some "down tilt" so it's sorta looking at the ground

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BTW, --weezl--, has anyone told you that your avatar pic looks like one of the popularly photographed mountain men who act as informal guides in Kluane National Park, Yukon, Canada?

there's one guy on one of the park websites that could be your avatar's missing identical twin.

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As far as using liquid for temp stabalization, the first question one of the guys who hadn't seen it before asked how we kept them from floating....

It's easy when the bottom 6" of the cavity is a solid block of lead

 

[--weezl--]

Consider where mst repeaters are sited, on Water towers or the 200foot level of a commercial FM or TV broadcast tower are real popular.


And one overlooked thing about repeater sites is that the antennae made for high mounted repeater ops are specifically made slightly off the ideal length.
The thing is if you take an antenna that slightly shorter (or was it longer?) than the ideal cut for frequency and you then sharply tun it onto frequency he annular antanna pattern will have some "down tilt" so it's sorta looking at the ground

***************************************

BTW, --weezl--, has anyone told you that your avatar pic looks like one of the popularly photographed mountain men who act as informal guides in Kluane National Park, Yukon, Canada?

there's one guy on one of the park websites that could be your avatar's missing identical twin.

***************************************

As far as using liquid for temp stabalization, the first question one of the guys who hadn't seen it before asked how we kept them from floating....

It's easy when the bottom 6" of the cavity is a solid block of lead

i did not know what about any of the points, the antenna chopping makes sense, but i don't know enough about ham to really understand anything totally in this thread...

as for the picture, no, no one has told me that... the picture isn't actually me... someone on the forum posted it up... there was actually about 3-4 pictures posted for me to use as my av, because at 1500 posts, i still hadn't put up an av yet... this was the best one, i think RSH found it, but i could be wrong...

 

[4x4junkie]

And one overlooked thing about repeater sites is that the antennae made for high mounted repeater ops are specifically made slightly off the ideal length.
The thing is if you take an antenna that slightly shorter (or was it longer?) than the ideal cut for frequency and you then sharply tun it onto frequency he annular antanna pattern will have some "down tilt" so it's sorta looking at the ground

I recall it wasn't the length of the antenna that does that, but more specifically from progressively feeding each of a vertical antenna's collinear elements very slightly out of phase. I can see how this would naturally occur on an antenna used a few MHz off from it's design frequency though, so you're probably correct as well.

 

[AllanD]

I recall it wasn't the length of the antenna that does that, but more specifically from progressively feeding each of a vertical antenna's collinear elements very slightly out of phase. I can see how this would naturally occur on an antenna used a few MHz off from it's design frequency though, so you're probably correct as well.

Often commercial repeater arrays are constructed by using offset pieces of coax so to BE out of phase the lengths must be "off"

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[gvfd29]

They make a "firestick" for ham users called the hamstick. I use two for 6m ssb.

 

[--weezl--]

ooooohhh! hamstick you say!?!? you just made me happy!

i can't seem to find any pics of anything that looks like the fire stick (single fiberglass antenna)