Sidebar Sidebar
  • You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Base Antenna Coax length,Best Coax and best Antenna under $250.00

Oh my!

bob85 said:
imax is more floppy than gainmaster

P1010138.jpg


a99 may be a little stiffer, i have seen several a99 and and imax shredded by wind, the one in the pic survived because the pole bent,
i don't know which is stronger until we get big winds hence my reservations.
Click to expand...

Nothing a new antenna or a new mast pole can't fix
 
Last edited:
Lil'Yeshua said:
I get queasy just looking at those YouTube vids showing the GainMaster moving all over the place. One of the vids I posted(the negative exposure one)showed a GainMaster mounted on a mast that swayed with the antenna and not contributing to the stress on the antenna at the choke coil.
Click to expand...

The survivability of an antenna should never rely on the mast bending or moving. An antenna should be build to withstand the wind/ice etc without relying on something else to give or fail to protect it. If it cannot do so then it is of inferior design. I have had large amateur band antennas survive hurricane force winds and gale force winds with icing. Never has the mast bent as I designed the installation to be over-strength.Most people, including some manufacturers, fail to understand just what the wind is capable of when it comes to antennas. Really long tall antennas that are mounted at the base are especially prone to breaking at the bottom. It is just the law of physics. Try and hold a four foot pole in the wind and then try and hold a twelve foot pole in the same wind. There is a HUGE difference. The antenna becomes a force multiplying lever with incredible forces applied to the base. It is for that reason I do not like really long vertical antennas like the Gainmaster and others. You can make the mounting structure bomb proof however you are at the mercy of the manufacturer understanding or lack thereof of one of the fundamental laws of physics.
 
  • Like
Reactions: 2 people
Lil'Yeshua said:
300 divided by frequency multiplied by vf = element length for 1/4 wave dipoles
Click to expand...

i used 492 divided by frequency , then added 2 inches when i made mine . then used a meter to trim it to tune . forget about velocity factor ...... make it a little long and use a vswr meter to tune it to your installation .....
 
Last edited by a moderator:
I'm afraid that's not how it's done, the answer is off by more than just a little bit.
I don't deal in the metric system so this is in feet, keep that in mind.
The 'magic' number to use for finding a 1/2 wave length is '468', which will get you very close to the required total length for a 1/2w dipole. The result will be a length that's very slightly 'too long', but only by a couple of inches, which is normal. Or, if you feel like you should use a velocity factor that 'magic' number changes to '492'. (That '468' already has the VF factored into it.) So;
468 / f(Mhz) = feet
468 / 27.200 Mhz = 17.2 feet (You can carry that out to as many decimal places as you want)
or-
(492 x 0.95vf) / 27.20 Mhz = 17.18 feet (a whole .02 of a foot or a 1/4 inch difference?)

You would still need to do some fine tuning with either length, and it's a good idea to add a foot or two to allow for making connections and wrapping around insulators etc.
But that's really about as close as you can get. There are just too many 'variables' with any antenna installation to get it any more accurate than that. That velocity factor will also change slightly with the diameter of the conductor used to make the antenna. The '0.95' that I used is the most commonly used VF for wire. As the conductor diameter gets larger the VF gets smaller.
The most commonly used 'magic' numbers are;
984 = 1 wave length
468 = 1/2 wave
234 = 1/4 wave

You might notice that those 'magic' numbers are not a linear progression. They will never be linear and the non-linearity shows up more with the increase in quantities being calculated. That deals with velocity factors and 'air'.
None of this is 'exact', and it doesn't explain where those 'magic' numbers come from. If you want to know that then I'd suggest finding a good text. There are a number of them, the ARRL Hand Book is one of them, also one by William Orr (his explains more too).
- 'Doc
 
Captain Kilowatt said:
The survivability of an antenna should never rely on the mast bending or moving. An antenna should be build to withstand the wind/ice etc without relying on something else to give or fail to protect it. If it cannot do so then it is of inferior design. I have had large amateur band antennas survive hurricane force winds and gale force winds with icing. Never has the mast bent as I designed the installation to be over-strength.Most people, including some manufacturers, fail to understand just what the wind is capable of when it comes to antennas. Really long tall antennas that are mounted at the base are especially prone to breaking at the bottom. It is just the law of physics. Try and hold a four foot pole in the wind and then try and hold a twelve foot pole in the same wind. There is a HUGE difference. The antenna becomes a force multiplying lever with incredible forces applied to the base. It is for that reason I do not like really long vertical antennas like the Gainmaster and others. You can make the mounting structure bomb proof however you are at the mercy of the manufacturer understanding or lack thereof of one of the fundamental laws of physics.
Click to expand...


and there's the option of building what you want to survive your areas weather extremes .
 
  • Like
Reactions: 1 person
Yep

I seen that about the ARRL handbook about building antennas. With tall verticals it's only a matter time before part of it fatigues and breaks. Does Sirio have any info on how many cycles their Gain Master goes through before the the vertical antenna cracks from fatigue? I've checking out the inverted dipole. Or at at a l-shaped dipole or some other configuration of a dipole. My tower's 1.25" galvanized pipe mast extends 6' out of the top of the tower. Two long distance stacked tv antennas are mounted on the mast with a 1/4 wave whip on top. I have tall pines shielding most of the wind from it but I have seen the mast/antennas buffeted right smartly but with a water pipe for a mast,it holds it's own. A tall vertical antenna would be vulnerable to high winds. A heavy duty set of beams would have similar performance as my tv antennas. If the Gain Master has thicker wall material near it's base with it tapering towards the top then it might last a while. The guy on YouTube that had the broken GM probably had a freak blast of turbulent wind causing his to break. It does look like his mast is bent slightly.
 
Lil'Yeshua said:
300 divided by frequency multiplied by vf = element length for 1/4 wave dipoles
Click to expand...


First thing is that there is no such thing as a 1/4 wave dipole. Perhaps you mean a 1/2 wave dipole with each side being 1/4 wavelength long. The other thing is that number you used, 300, is the number to use when trying to determine the overall wavelength in meters. 300 ÷ 27.0 = 11.111 meters long. About 36 1/2 feet. A dipole would be 1/2 that over all or about 18 1/4 feet. Each side would be 9 feet 1 1/2 inches.
 
Just an FYI. My antenna masts have always been steel pipe 1.9 inches O.D. and there has been very little bending except in extreme winds with a heavy top load. Most of the big antennas have been mounted just above the tower top so as to transfer the forces to the tower and eliminate flexing of the mast. The new mast which will be populated with antennas in the spring is now 2 inch inside diameter pipe 21 feet long of which 17 feet will be above the top of the tower. The Hygain Ex-14 yagi will be one foot above the tower while the Cushcraft 12/17m yagi will be about 5 feet from the top. There will be a pair of Cushcraft 13B2's at approx. 7 and 17 feet from the top and a six element 6m yagi in the middle of the 13B2's at 12 feet above the tower. The mast as said is 2 inches inside dia. which is reinforced with a heavy duty 8 foot galvanized steel mast which came with the tower inside it and positioned so it extends 1 foot below and 7 feet above the top of the tower which is the point of failure for any long mast. Inside this is another 4 foot length of aluminum pipe making the entire mast 1/2 inch thick at the point of failure. No thin 1.25 inch tubing would ever stand even just the Hygain Explorer14 if mounted even 6-7 feet high on it. The antennas will take pretty much whatever Mother Nature will throw at them here and I have made the mast capable of doing the same thing. This is what I meant by not underestimating the wind and that an antenna should never rely on a mast failure to survive. To do so is irresponsible and idiotic.The tower top is 62 feet above ground BTW so it will catch all kinds of wind.
 
have not heard or seen any bad info on the gainmaster and they have been using them across the pond for loner than they have been here. had some 40-50mph wind here today, came on very fast and i just watched it swing
 
Mast

Captain Kilowatt said:
Just an FYI. My antenna masts have always been steel pipe 1.9 inches O.D. and there has been very little bending except in extreme winds with a heavy top load. Most of the big antennas have been mounted just above the tower top so as to transfer the forces to the tower and eliminate flexing of the mast. The new mast which will be populated with antennas in the spring is now 2 inch inside diameter pipe 21 feet long of which 17 feet will be above the top of the tower. The Hygain Ex-14 yagi will be one foot above the tower while the Cushcraft 12/17m yagi will be about 5 feet from the top. There will be a pair of Cushcraft 13B2's at approx. 7 and 17 feet from the top and a six element 6m yagi in the middle of the 13B2's at 12 feet above the tower. The mast as said is 2 inches inside dia. which is reinforced with a heavy duty 8 foot galvanized steel mast which came with the tower inside it and positioned so it extends 1 foot below and 7 feet above the top of the tower which is the point of failure for any long mast. Inside this is another 4 foot length of aluminum pipe making the entire mast 1/2 inch thick at the point of failure. No thin 1.25 inch tubing would ever stand even just the Hygain Explorer14 if mounted even 6-7 feet high on it. The antennas will take pretty much whatever Mother Nature will throw at them here and I have made the mast capable of doing the same thing. This is what I meant by not underestimating the wind and that an antenna should never rely on a mast failure to survive. To do so is irresponsible and idiotic.The tower top is 62 feet above ground BTW so it will catch all kinds of wind.
Click to expand...

That's one serious tower. And yes,there are faster wind speeds with altitude. I originally designed mine with only tv antennas in Mind. The tower can support a car. However, in order for it to handle a anything substancial I would have to modify the tower top bearing collar by increasing it's size to receive at least a two inch mast. It's a homemade deal that only cost me $250 per section to make. If I do modify it I would make it to have a telescoping tower section inside it thereby extending the tower's present height from 32' to 40 to 50'. Is there such a thing as beam antennas that include 11 meters?
 
I don't think there are any ham antennas that include an 11 meter element, there's just not much call for that. I would assume that a multi-band beam that includes 10 meters could be modified to 11 meters though.
- 'Doc
 

dxChat
Help Users
  • No one is chatting at the moment.


      Top Bottom
      Back