Designing custom antenna for my very specific local needs?

2870 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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Dave Loudin

· #21 · Sep 11, 2010

The equation looks like:

NM + antenna gain - preamp noise figure - cable loss > 10. All expressed in dB

If there is a pre amp, then cable loss = 0. We have talked about noise figures and cable loss specs elsewhere. Your approach is exactly what TVFool is designed for.

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nikiml

Discussion Starter · #22 · Sep 11, 2010

I somehow hoped that if :

NM + antenna gain - preamp noise figure >0 (preamp means no cable loss)

it will be ok. I thought that the preamp will take the (>0) amount of signal and make it tv acceptable.

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majortom

· #23 · Sep 11, 2010

Holl_ands provided some very useful information, the last time I brought up Link Budgets in this forum.

http://www.digitalhome.ca/forum/showthread.php?p=1064634&highlight=Link+BudgeT#post1064634

Another thing ya need to consider is the most basic link budget
of rcv'd Signal Strength - Noise Floor having to be greater than 15.2 dB C/N,
for ATSC doesn't account for any interference. Might be OK if the only limitation is Noise,
but we all know in reality that's hardly ever the case.
signals can bend & refract, causing them to propagate further than "predicted" one day,
and less than predicted the next day or even a few hours later.
Throw in tropo affects, and gets even more messy.

here's another link which might be useful.

http://www.google.com/url?sa=t&sour...eIhZRg&usg=AFQjCNGdUB2Itri5mpYTs00fYdUke0SSiA

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jflarin

· #24 · Sep 12, 2010

ganging.

nikiml said:
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.

Hi,

I see that this thread has shifted to "how to get a very weak signal", but when I read your initial message I though about something else.

One way to customize your antenna to get signal from different azimuth is to gang antenna (ok, I know there is a big difference between ganging and stacking and I hope I'm not using the wrong expression...) If you combine multiple antenna (or is it stack? I'll use combine instead of the wrong expression), you will get something that will give you 3 dB more in the main direction and secondary lobe and nulls depending upon the distance between and antenna and the frequency. (check http://www.hdtvprimer.com/ANTENNAS/ganging.html)

So here is what I would do (and will do when Canada switch to full digital): First use a good antenna (SBGH, M4 for example?) and find out all the channel you can get from every direction. If you can't get a channel at all by orienting a good antenna directly on it, I wouldn't hope that the 3dB you can get by combining antenna will give you a reliable signal.

After that, make a list of the hardest to get channels. In my case (http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d9c6b4c74416ac9), it is the 13 (real) and 43 real. Next the 27 is difficult go get because it is at 55 degrees to the left than everything else. The other channel that resides 30 to the right is so strong (+38 dB of NM) and I don't care.

So according to the ganging chart shown in the URL above, if I put to antennas at about 27 inches center to center, I will produce a secondary lobe about 55 degrees from the maximal gain of my antenna. So I'll orient them both toward the weak signal (43 and 13), get almost 3 dB of gain for those and still get channel 27 (real). But to get those benefit, both antenna have to be the same, which may be difficult with DIY built antenna.

The other thing I want to explore is the X-GH8. This antenna is optimized for a single channel, getting a few db (up to 4?) above the GH10, but for a very narrow band. My hope is that I could model (and then build) an hybrid that will give the most gain on 43, something comparable to GH10 on 33 and less gain on 14 and 22 (which I don't have any problem getting anyways).

jf

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