[OKDXA] VU7

Sat Dec 9 21:28:43 EST 2006

Bert,
Thanks for all the info.  All things considered I'm just at a distinct 
disadvantage with my antenna being the height it is and there's nothing I 
can do about that.  I guess I'll just have to hope for some freak opening or 
something.
Gene
----- Original Message ----- 
From: "Bert Aaron" <k2ba at cox.net>
To: "'Discussion of OKDXA'" <okdxa at mailman.qth.net>
Sent: Saturday, December 09, 2006 12:55 PM
Subject: RE: [OKDXA] VU7


> Gene,
>
> Since you have not worked them yet, you don't know if they are hearing 
> you.
> There are many factors that could affect the ability to receive them and
> vice versa. In general the properties of antennas are reciprocal. The gain
> on receive is the same as on transmit. The noise floor at each end of the
> path will have a big impact on your hearing them.
>
> What follows is an example of how the signal is computed by propagation
> programs. The standard level used for signal strength is dBm (decibel
> reference to a milliwatt). Zero dBm is a milliwatt. If the power output of
> the transmitter is 1 KW then it is a million times greater than a 
> milliwatt.
> This is a 60 dB increase in the signal or 60 dBm. Ignoring loss in the 
> coax
> we add the gain of the antenna, +15 db. We are now at 75 dBm. Next 
> subtract
> the path loss. VOACAP predicts the loss at 194 dB on the long path. The
> signal level is now -119 dBm. The published sensitivity of the receiver
> might be in the -131 dBm range however, that is into a 50 ohm dummy load.
> When you connect an antenna to the receiver the noise floor rises to the
> level of your local environment. In order to receive the signal you 
> probably
> need signal to be 10 dB above the noise. In the above example you need a
> signal of at least -109 dBm to copy him. If your local noise floor, with 
> the
> antenna on is 12 dB above the internal noise of the receiver (-131 dBm),
> then you won't hear him. We assumed above an antenna gain of +15.0 dBi but
> it is much less. In your case it is -0.5 dB. That means that the signal
> level had dropped 14.5 dB, which probably puts it well below the noise
> floor. This is assuming that the antennas are the same gain and height at
> both ends of the path. This is probably not the case. My understanding is
> that they are using a 4 element SteppIR on the three of the stations and a
> vertical SteppIR on the fourth. I don't know how high the antennas are.
> There are a lot of variables. The path loss changes, the arrival angle
> changes, the noise floor at each end changes, etc. From listening to them 
> it
> appears that they are hearing very well so I assume their local noise 
> floor
> to be low. I don't know what your noise floor is. The short path arrival
> angle is higher than long path so if and when you work them it probably 
> will
> be on the short path.
>
> I hope I have explained what is happening. If you have any questions, ask
> away.
>
> Bert K2BA
>
> -----Original Message-----
> From: okdxa-bounces at mailman.qth.net [mailto:okdxa-bounces at mailman.qth.net]
> On Behalf Of Gene Lewis
> Sent: Saturday, December 09, 2006 12:07 PM
> To: Discussion of OKDXA
> Subject: Re: [OKDXA] VU7
>
> I follow what you're saying about the plotting of the gains and all but 
> how
> exactly does that affect reception?  I'm not having a problem transmitting 
> a
>
> signal to them, I'm having a problem hearing them.  How does all these
> numbers affect reception?  Or am I dense?
> Gene
>
> ----- Original Message ----- 
> From: "Bert Aaron" <k2ba at cox.net>
> To: <n5ok at arrl.org>; "'Discussion of OKDXA'" <okdxa at mailman.qth.net>
> Sent: Saturday, December 09, 2006 11:56 AM
> Subject: RE: [OKDXA] VU7
>
>
>> Coy,
>>
>> At 75 feet the gain is 15.25 dBi at 13 degrees and 3.0 dBi at 2 degrees.
>>
>> Bert  K2BA
>>
>> -----Original Message-----
>> From: okdxa-bounces at mailman.qth.net 
>> [mailto:okdxa-bounces at mailman.qth.net]
>> On Behalf Of Coy Day
>> Sent: Saturday, December 09, 2006 11:49 AM
>> To: Discussion of OKDXA
>> Subject: RE: [OKDXA] VU7
>>
>> Bert,
>>
>> Did you happen to check it at 75ft?  A lot of city dwelling hams use that
>> as the optimum height for 20 meters.  Just curious what another 20ft 
>> would
>> do.
>>
>> Coy
>> -- 
>> Coy Day, N5OK
>> 20685 SW 29
>> Union City, OK 73090
>> 405-483-5632
>>
>> Bert Aaron wrote:
>>> Gene,
>>>
>>> Just ran a plot of two 5 element Yagi's, over flat terrain, one at 151
>>> feet
>>> and the other at 55 feet. According to HFTA the antenna at 55 feet has a
>>> gain of 15.0 dBi at 17 degrees and a gain of -0.5 dBi at 2 degrees. The
>>> same
>>> antenna at 151 feet has a gain of 15.5 dBi at 7 degrees and a gain of 
>>> 7.5
>>> dBi at 2 degrees. That is a difference of 8.0 dB due to the change in
>>> height, at 2 degrees, and probably explains the problem.
>>>
>>> 73,
>>>
>>> Bert  K2BA
>>>
>>> -----Original Message-----
>>> From: okdxa-bounces at mailman.qth.net
>>> [mailto:okdxa-bounces at mailman.qth.net]
>>> On Behalf Of Gene Lewis
>>> Sent: Saturday, December 09, 2006 9:53 AM
>>> To: n5ok at arrl.org; Discussion of OKDXA
>>> Subject: Re: [OKDXA] VU7
>>>
>>> Roger that, Coy,
>>> Sure wish I could run the antenna on up a ways but don't have that
>>> option...heavy sigh 8-<
>>> ----- Original Message -----
>>> From: "Coy Day" <n5ok at arrl.org>
>>> To: "OKDXA" <okdxa at mailman.qth.net>
>>> Sent: Saturday, December 09, 2006 9:48 AM
>>> Subject: [OKDXA] VU7
>>>
>>>
>>>> Gene,
>>>>
>>>> Bert is absolutely correct in that the take off angle for that part of
>>>> the
>>>> world is absolutely critical.  On 20 meters with the antenna at 55ft
>>>> your
>>>> take off angle is way too high.  You would have a better chance on 15 
>>>> if
>>>> the band would open.  :-(
>>>>
>>>> Coy
>>>> --
>>>> Coy Day, N5OK
>>>> 20685 SW 29
>>>> Union City, OK 73090
>>>> 405-483-5632
>>>>
>>>>
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>>>>
>>>
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>>
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