2
$\begingroup$
  1. We have a VHF Omni directional antenna, 2.5 W and 119 MHZ (approximate) mounted on top of a commercial hanger. Access to the antenna is out of the question because of the lack of fall protection. Service personnel have expressed apprehension over accessing the antenna.

    enter image description here

  2. We’re trying to come up with a solution that minimizes the risk to service personnel

  3. One idea is using two VHF antennas mounted at 8 feet elevation at diagonally opposite sides of the hanger with a Wilkinson Power-Divider at the VHF transmitter to ensure that the two antennas are properly phased. The VHF Antennas would be mounted at a much lower elevation (8’) on each side of the Hanger.

  4. Two antennas are used because of the obstruction caused by the hanger when mounting them at 10'

a. Do you have any thoughts on this? b. Do we have a reasonable chance of success? c. Is this idea of any merit?

enter image description here

$\endgroup$
  • $\begingroup$ Please state the problem you're trying to solve. What's wrong with the antenna you already have? What's the obstruction? Is it the hanger? $\endgroup$ – mrog Oct 16 '19 at 16:10
  • $\begingroup$ Technicians are apprehensive of going up there. There are no railings or handholds or tie offs. The surface is deteriorating. Its pretty dangerous. $\endgroup$ – Cof Fee Oct 16 '19 at 16:37
  • 2
    $\begingroup$ Possible duplicate of Two antennas on different sides of a building $\endgroup$ – Phil Frost - W8II Oct 16 '19 at 17:27
  • 1
    $\begingroup$ 6. A major concern is that an unexpected beam pattern would result. a. We don’t know if the ideal result, shown below, would result. b. We could have a beam pattern with some unintended results. $\endgroup$ – Cof Fee Oct 16 '19 at 17:42
  • 2
    $\begingroup$ @CofFee yes, but you start of saying you're "looking for a solution" without saying what you're looking for a solution to, and you want to know your "chance of success" without defining what "success" means. What does the existing antenna not do that it should? Why are you contemplating new antennas? How will you know if you succeed? $\endgroup$ – hobbs - KC2G Oct 16 '19 at 20:52
3
$\begingroup$

It sounds like the current antenna isn't working, and you want to replace it without climbing on the roof, right? I recommend installing a single antenna on the highest point you can reach using a lift or tall ladder, without setting foot on the roof. If needed, you can use a tall mast to elevate the antenna so it's higher than the roof peak. This would avoid the potential pitfalls of using two antennas (including reduced gain and destructive interference).

| improve this answer | |
$\endgroup$
  • 2
    $\begingroup$ A mast or tower clamped to the building, absolutely is the way to go. $\endgroup$ – Mike Waters Oct 16 '19 at 19:00
  • 1
    $\begingroup$ I guess I should clarify I apologize but I can't believe I didn't add this. We are a State Agency and the building is a Private Property . They let us host a radio advisory system that benefits them. We just can't put in masts, or extensive modifications because its private property and we can't spend hi amounts of taxpayer money on a commercial l building so we are tied to $5K as a limit. $5K these days will buy you a lot of RF cable, fittings, an extra antenna, hardware and that's it. A tilt down tower would be $10 K because we have to follow the code to install it. $\endgroup$ – Cof Fee Oct 18 '19 at 3:00
  • $\begingroup$ I've thought about every workaround I can, including a home made rail system or a hoist or a folding mast, but the bottom line is this is a private property.There is a liability. So we have to keep the signature of what we do light. That's why I am posing the question of 2 antennas. Success would be maybe reaching the same coverage we have now with only a few dead zones, vice huge amounts of dead zones. $\endgroup$ – Cof Fee Oct 18 '19 at 3:12
  • $\begingroup$ Thanks everyone. I am very inexperienced in RF. I am only Google smart/ $\endgroup$ – Cof Fee Oct 18 '19 at 3:12
  • $\begingroup$ @CofFee an appropriate mast will cost less than $20. Example: amazon.com/dp/B07C95LLBT $\endgroup$ – Phil Frost - W8II Oct 24 '19 at 17:51
2
$\begingroup$

When you feed two antennas with equal power and the separation is large you would create a pattern with very deep nulls at points where both antennas are visible.With the proposed arrangement that would be two 90 degree sectors with problematic coverage. A better solution would be to use the hanger as a screen to minimize the sectors where both antennas are seen equally well. That would be at the midpoints of the long sides and close to the vertical wall. Maybe below the lower roof in case you do not need the range associated with more antenna height. Cable lengths do not matter and you should use a simple power splitter because in case there is any unbalance there is nothing to gain by heating the resistor in a Wilkinson splitter/combiner. 119 MHz seems to me like aeronautical. If that is the case so the signal arrives from some elevation, use a low height and do your best to minimize the directions from which both antennas are visible - but of course high enough to avoid obstacles that might screen the signal.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.