Designing custom antenna for my very specific local needs?

2873 Views 23 Replies 9 Participants Last post by jflarin, Sep 12, 2010

nikiml

Discussion Starter · Sep 9, 2010

This is a question to all the OTA gurus on this forum.

I would like to design custom antenna for my location,
based on the numbers given by tvfool.

The weakest channel I'd like to get is 23 (ION) and for this I'd probably need a preamp.

By looking at the tvfool chart I've come up with a list of frequencies, azimuths and target gains.

My question are:

Are the target gains enough (with /without preamp)?
Are some if target gains to much if I am to use a preamp?
Could I go with less gain on any channel?

Code:

Chan	Network	Dist	Path	NM 	azim     target gain
	        (mi)            (dB)	(deg)    (dbi)
-------------------------------------------------------
20	CBC	19.8	LOS	49.6	50	    0
18	Ind	26.2	LOS	46.7	-50	    0
40		19.8	LOS	44.4	50	    0
44	OMN	20.2	LOS	43.1	50	    0
64	OMN	20.2	LOS	41.6	50	    0
35	Ind	26.2	LOS	38.6	-50	    0
24	SRC	19.8	LOS	37.4	50	    0
32	CW	57.6	LOS	37.2	0	    0
65	GTN	19.8	LOS	34.9	50	    0
66	Sun	19.8	LOS	34.8	50	    0
53	Ind	19.8	LOS	33.5	50	    0
43	PBS	57.6	LOS	29.5	0	    1
14	Fox	57.5	LOS	24.2	0	    1
39	CBS	86.2	LOS	21.7	-5	    1
33	NBC	85.4	1Edge	17.5    0	    5
38	ABC	87.5	1Edge	12.9	-5	    8
26	Ind	87.1	1Edge	11.5	-30	    9
49	MyN	86.3	1Edge	7.7	0	    10
7	Ind	88.0	2Edge	-7.2	-10	    8
23	ION	100.2	2Edge	-13.5	10	    16

Thanks in advance,

nikiml

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PCmonkey

· #12 · Sep 9, 2010

Hi Nikiml,

If I understand you correctly, it looks like you are trying to spec an antenna that has just the right amount of gain at the channels/frequencies of interest...so for the far away stations you want lots of gain and for the locals you want reduced gain to prevent overload...correct?

If so, I think the first step would be to graph this. Maybe split it into major directions (CN Tower, Buffalo). Graph frequency vs expected signal strength. Look at the graph to see how gradual the signal strength numbers change - I think it will be nearly impossible to design a single antenna if you require gain notches at multiple frequencies...
Not sure if this helps...

T2

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PCmonkey

· #16 · Sep 10, 2010

So here's my best guess:

I believe that the input sensitivity of a typical ATSC tuner is around -83dBm...
TV Fool lists the signal power of the various stations in dBm as well.

So to determine the needed gain of an antenna you need bring the signal up to a power level that falls within the sensitivity of the receiver (also need to compensate for some losses from the balun, connectors, etc.)

For example from my location:
TV Fool power estimate for WKBW-DT: -90dBm
My Antenna gain: 14dBi

Resultant signal output power from antenna: -90dBm + 14dB = -76dBm
Assuming there aren't significant losses in your system, then this should fall within the sensitivity range of your ATSC tuner (-83dBm).

Things to remember:
- decibels add (so math is easy)

- dBm = ratio of power in mW with respect to 1mW
for example: 0dBm = 1mW, 3dBm = 2mW, -3dBm = 0.5mW
- dBi = isotropic gain of the antenna.

EXPERTS - please correct me if I've messed up - this isn't my exact field of study

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PCmonkey

· #18 · Sep 10, 2010

>21.7 dBi sounds right - That channel is 160km away - I'd say that's going to be difficult.

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