[OKDXA] VU7

Sat Dec 9 13:55:23 EST 2006

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
>>>
>>>
>>> _______________________________________________
>>> OKDXA mailing list
>>> OKDXA at mailman.qth.net
>>> http://mailman.qth.net/mailman/listinfo/okdxa
>>>
>>
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>
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