Modify CM-3672 ? - Canadian TV, Computing and Home Theatre Forums

kc2cbd · 2011-09-12, 12:39 AM

I have an old Channel Masters Crossfire CM-3672 I believe.
I would like to get some feedback on removing the uhf portion.
About 4 or 5 months ago refurbished the antenna and since have acquired a CM-4251 so want to modify the Crossfire for VHF only.
My idea was to remove the matching bars and uhf reflectors.
I originally thought I was going to be able to remove the whole uhf section but there are vhf elements on that section.
Does anyone think removing the uhf elements themselves will hurt the antenna for vhf only?
If not I wanted to use those to make a small uhf antenna to point in the opposite direction for a local station that is almost 180 degrees from my Nashville stations.

Here is my antenna.

There are 2 other photos here
https://picasaweb.google.com/1069450...eat=directlink

My other question is will running 300ohm twinlead from this antenna to the CM-4251 then from those lugs attach a balun which will then go to my Winegard AP-8780. Will this work or will it hurt reception?
I figure that I would be better off getting a UVSJ to combine them.

Any comments appreciated.

300ohm · 2011-09-12, 08:59 PM

Quote:

My idea was to remove the matching bars and uhf reflectors.
I originally thought I was going to be able to remove the whole uhf section but there are vhf elements on that section.

Correct, you have a vhf-hi director on the uhf boom that you should leave in place.
There may be vhf-hi directors on the corner reflector, Im not sure. How long are those longest reflectors on the corner reflector ?

Ive modeled taking off the uhf sections (and vhf-low elements ) on antennas like the Radio Shack VU series, which are similar LPDA's with the cross over phasing lines, like yours. Slicing and dicing works out well with those type of antennas. Double boom or vee boom type LPDAs are a different story.

brantford bill is taking off the uhf section of a Channel Master kind of like yours, but a bit smaller. Starting with post 551 here :
http://www.digitalhome.ca/forum/show...107706&page=37

He seems to have run into a problem. Whether the balun is broken or that he needed to leave the flat metal folded dipole (in your case, that would be the flat metal bowtie) on because it serves as a termination device, I dont know.

kc2cbd · 2011-09-12, 09:44 PM

I don't see any vhf directors on the reflector booms.
If you look at my Picasa photo album I linked to you will see that my antenna has matching tubes instead of wires like Brantford_Bill's antenna.
Although I'm baffled as to what those bars are for on his UHF folded dipole are.
I'm assuming it attaches to another dipole or something.
I think if I remove my matching tubes and connect to the VHF dipole connection point instead of the UHF connection point that I connect to right now I think mine will work that way.

My other question is will the connection from there to the CM-4251 then subsequently to the pre-amp via a balun. will it work and not degrade the signal of the combining of the 2 antennas?

300ohm · 2011-09-12, 09:55 PM

Quote:

My other question is will the connection from there to the CM-4251 then subsequently to the pre-amp via a balun. will it work and not degrade the signal of the combining of the 2 antennas?

I dont see a preamp in the pictures, so join the two with a UVSJ (Uhf-Vhf-Signal-Joiner). Yes, youll need a balun on both antennas, and some losses will occur. But the extra gain of the CM4251 will more than overcome those on uhf.

Quote:

My other question is will running 300ohm twinlead from this antenna to the CM-4251 then from those lugs attach a balun which will then go to my Winegard AP-8780. Will this work or will it hurt reception?

That wont work out as well, so use the UVSJ. (you could experiment with 450 ohm ladder twin lead if you have some)

kc2cbd · 2011-09-12, 10:22 PM

I don't have any 450ohm ladder available...I do have a few feet of 300 ohm from when I acquired the 4251 and another old really beat up CM antenna which I disassembled.
I guess I will see if I can acquire a UVSJ locally, if not I will have to order one and wait for it.
I do have a preamp but it isn't in those pictures as I hadn't acquired it yet when I took those pictures.
In fact that antenna is now about 10 feet higher than it is in those pictures and has 3 guide wires attached. It is also before I refurbished that antenna too.
When I refurbished the antenna and added the pre-amp and raised it up I increased my reception.
What I'm trying to achieve is get better reception on channel 23 (virtual 58.1 & 58.2) which doesn't come in reliably right now.
My other issue is I need the VHF low and hi cause I recieve 5, 8 & 10 so I need to keep the VHF portion of this antenna.
Finding VHF only antennas for 2 to 13 are few and far between and cost more than I'm willing to spend right now.

This antenna feeds 2 television and a Hdhomerun reciever for my Mythbox.

300ohm · 2011-09-12, 10:33 PM

Quote:

I guess I will see if I can acquire a UVSJ locally, if not I will have to order one and wait for it.

Another option for slightly less loss is to use a Winegard SD3700 300 ohm to 75 ohm outdoor coupler. http://www.solidsignal.com/pview.asp...-3700_Winegard

Quote:

What I'm trying to achieve is get better reception on channel 23 (virtual 58.1 & 58.2) which doesn't come in reliably right now.

Post your TVFool. I dont want to burst your bubble, but it seems like the CM4251 has a dip in the gain curve at channel 23, so it may not do much better than the uhf section of the CM3672. Post 6 here has Channel Masters tested gain plot.
http://www.digitalhome.ca/forum/show...ghlight=cm4251

kc2cbd · 2011-09-12, 10:53 PM

I live in Cookeville, TN
I can't tell if the dip is worse than my existing antenna.
If it is just a little better I would be happy.
If it ends up worse than I will make the wife very unhappy as she is happy with it now.

300ohm · 2011-09-12, 10:55 PM

Ideally, what you want is a CM0264 preamp like what I have. (seem to be hard to get now) It has separate 300 ohm inputs for uhf and vhf, and a 75 ohm output, low noise and durable.

kc2cbd · 2011-09-12, 11:12 PM

Probably but I got this Winegard AP-8780 and haven't been disappointed.
I think I have a local source for the UVSJ that is if Kmart has them at the store. If not I don't know where I can get one locally and will probably order 1 or 2 from SolidSignal which seems to be a good price of less than 2 dollars each.

kc2cbd · 2011-09-13, 09:00 PM

I added some more pictures to my picasa album that might help make a better assessment.
Appreciate your help 300ohm
I didn't find a UVSJ locally so I'm going to have to order one...so might order a couple or more considering SolidSignal has them pretty cheap.

300ohm · 2011-09-23, 11:16 PM

Quote:

Oh yeah those elements were mounted on plastic spacers.
I still have those spacers and plan to re-use them.

Great. Can you post a picture of the director pair mounted on the plastic and the dimensions of the air space between the pair, and the air space as it would have been over the boom. I think they would have been on the same up/down plane as the other elements.

Quote:

Far as I can tell the UHF section is different but I don't know about dimensions.

I put a picture of mine and a picture of a CM-3671 in my albums.

Yeah, the vhf section of the CM-3671 with the UHF pieces gone looks like your CM-3672 vhf section. Its hard to tell from the photos, but compare you have to what I have of the VHF section of the CM-3671 model with the UHF pieces removed :

I put the letters and numbers in there for future reference. The red dot is the balun connection point. The spacing at A, the distance between the last front active element and the first director, is 14 inches measured center of tube to center of tube. Can you compare this to yours ? Its a pretty critical dimension and hopefully yours is very close to the same distance.

Quote:

Was just wondering if those elements would help the signal or would I be better off just using my other straight elements.
I have plenty of extra aluminum elements from 2 antennas that I disassembled.

Duplicating extra directors like on the CM-3671 looks pretty darn good to me. When I took off the UHF elements from the CM-3671 model, the Net Gain increased over 1.1 dBi on channel 10, mostly due to much lower SWR. Low SWR is more important for digital than it was for analog TV. Raw Gain on channel 10 - 11.7 dBi SWR 1.25 Net Gain 11.6 dBi. Compare to Net Gain of about 10.5 dBi with the UHF elements in place here :
http://www.hdtvprimer.com/ANTENNAS/comparing.html

Quote:

Does anyone think removing the uhf elements themselves will hurt the antenna for vhf only?

Pretty sure that question is answered, heh.

300ohm · 2011-09-23, 11:28 PM

The NEC file:

Code:

CM CM3671 :  Channel Master 3671B, converted with 4nec2 on 25-Nov-08 12:47
CM Ken Nist CM-3671 model with UHF relevant pieces removed by 300 ohm
CM AGT = .99 (0db) at 195mhz, Auto Segmentation = 21
CE
GW	8	75	0.95	53.8	0.5	0.95	-53.8	0.5	0.1875
GW	9	67	13	-47.7	0.5	13	47.7	0.5	0.1875
GW	10	61	24.9	44.15	0.5	24.9	-44.15	0.5	0.1875
GW	11	59	37	-41.6	0.5	37	41.6	0.5	0.1875
GW	12	55	49	38.6	0.5	49	-38.6	0.5	0.1875
GW	13	49	61	-35.6	0.5	61	35.6	0.5	0.1875
GW	14	45	73	32.2	0.5	73	-32.2	0.5	0.1875
GW	15	41	85	-29.15	0.5	85	29.15	0.5	0.1875
GW	16	1	92	0.3	0.5	92	-0.3	0.5	0.075
GW	17	5	97	1.8	0.8	92	0.3	0.5	0.075
GW	18	5	97	-1.8	0.8	92	-0.3	0.5	0.075
GW	19	17	97	1.8	0.8	97	26.6	0.8	0.075
GW	20	17	97	-1.8	0.8	97	-26.6	0.8	0.075
GW	23	23	4.85	16.1	1.2	4.85	-16.1	1.2	0.1875
GW	24	23	16.9	15.6	1.2	16.9	-15.6	1.2	0.1875
GW	25	21	28.5	14.5	1.2	28.5	-14.5	1.2	0.1875
GW	26	21	40.5	14.3	1.2	40.5	-14.3	1.2	0.1875
GW	27	19	52.5	13.5	1.2	52.5	-13.5	1.2	0.1875
GW	28	19	64.5	13	1.2	64.5	-13	1.2	0.1875
GW	29	19	76	13	1.2	76	-13	1.2	0.1875
GW	30	17	88	12.3	1.2	88	-12.3	1.2	0.1875
GW	31	17	99.9	12.1	-0.75	99.9	-12.1	-0.75	0.1875
GW	33	15	111	7	-0.7	111	28.2	-0.7	0.1875
GW	34	15	111	-7	-0.7	111	-28.2	-0.7	0.1875
GW	171	1	126.65	-6.5	1	126.65	-7.75	1	0.1875
GW	172	13	126.65	-8.5	1	126.65	-26.4	1	0.1875
GW	173	1	126.65	-7.75	1	126.65	-8.5	1	0.1875
GW	174	1	126.65	-7.75	1	127.74	-7.75	1	0.06
GW	175	1	126.65	-8.5	1	127.34	-8.5	1	0.06
GW	176	1	128.99	-12.3	1	128.24	-12.3	1	0.06
GW	177	1	128.99	-9.98	1	128.24	-10.14	1	0.06
GW	178	3	128.99	-12.3	1	128.99	-9.98	1	0.06
GW	179	1	128.24	-12.3	1	128.24	-10.14	1	0.06
GW	180	3	128.99	-9.98	1	127.74	-7.75	1	0.06
GW	181	1	128.24	-10.14	1	127.34	-8.5	1	0.06
GW	182	1	127.34	-8.5	1	127.74	-7.75	1	0.06
GW	183	1	126.65	-7.75	1	125.56	-7.75	1	0.06
GW	184	1	126.65	-8.5	1	125.96	-8.5	1	0.06
GW	185	1	124.31	-12.3	1	125.06	-12.3	1	0.06
GW	186	1	124.31	-9.98	1	125.06	-10.14	1	0.06
GW	187	3	124.31	-12.3	1	124.31	-9.98	1	0.06
GW	188	1	125.06	-12.3	1	125.06	-10.14	1	0.06
GW	189	3	124.31	-9.98	1	125.56	-7.75	1	0.06
GW	190	1	125.06	-10.14	1	125.96	-8.5	1	0.06
GW	191	1	125.96	-8.5	1	125.56	-7.75	1	0.06
GW	192	1	126.65	6.5	1	126.65	7.75	1	0.1875
GW	193	13	126.65	8.5	1	126.65	26.4	1	0.1875
GW	194	1	126.65	7.75	1	126.65	8.5	1	0.1875
GW	195	1	126.65	7.75	1	127.74	7.75	1	0.06
GW	196	1	126.65	8.5	1	127.34	8.5	1	0.06
GW	197	1	128.99	12.3	1	128.24	12.3	1	0.06
GW	198	1	128.99	9.98	1	128.24	10.14	1	0.06
GW	199	3	128.99	12.3	1	128.99	9.98	1	0.06
GW	200	1	128.24	12.3	1	128.24	10.14	1	0.06
GW	201	3	128.99	9.98	1	127.74	7.75	1	0.06
GW	202	1	128.24	10.14	1	127.34	8.5	1	0.06
GW	203	1	127.34	8.5	1	127.74	7.75	1	0.06
GW	204	1	126.65	7.75	1	125.56	7.75	1	0.06
GW	205	1	126.65	8.5	1	125.96	8.5	1	0.06
GW	206	1	124.31	12.3	1	125.06	12.3	1	0.06
GW	207	1	124.31	9.98	1	125.06	10.14	1	0.06
GW	208	3	124.31	12.3	1	124.31	9.98	1	0.06
GW	209	1	125.06	12.3	1	125.06	10.14	1	0.06
GW	210	3	124.31	9.98	1	125.56	7.75	1	0.06
GW	211	1	125.06	10.14	1	125.96	8.5	1	0.06
GW	212	1	125.96	8.5	1	125.56	7.75	1	0.06
GW	213	1	137.13	-6.5	1	137.13	-7.75	1	0.1875
GW	214	13	137.13	-8.5	1	137.13	-26.4	1	0.1875
GW	215	1	137.13	-7.75	1	137.13	-8.5	1	0.1875
GW	216	1	137.13	-7.75	1	138.22	-7.75	1	0.06
GW	217	1	137.13	-8.5	1	137.82	-8.5	1	0.06
GW	218	1	139.47	-12.3	1	138.72	-12.3	1	0.06
GW	219	1	139.47	-9.98	1	138.72	-10.14	1	0.06
GW	220	3	139.47	-12.3	1	139.47	-9.98	1	0.06
GW	221	1	138.72	-12.3	1	138.72	-10.14	1	0.06
GW	222	3	139.47	-9.98	1	138.22	-7.75	1	0.06
GW	223	1	138.72	-10.14	1	137.82	-8.5	1	0.06
GW	224	1	137.82	-8.5	1	138.22	-7.75	1	0.06
GW	225	1	137.13	-7.75	1	136.04	-7.75	1	0.06
GW	226	1	137.13	-8.5	1	136.44	-8.5	1	0.06
GW	227	1	134.79	-12.3	1	135.54	-12.3	1	0.06
GW	228	1	134.79	-9.98	1	135.54	-10.14	1	0.06
GW	229	3	134.79	-12.3	1	134.79	-9.98	1	0.06
GW	230	1	135.54	-12.3	1	135.54	-10.14	1	0.06
GW	231	3	134.79	-9.98	1	136.04	-7.75	1	0.06
GW	232	1	135.54	-10.14	1	136.44	-8.5	1	0.06
GW	233	1	136.44	-8.5	1	136.04	-7.75	1	0.06
GW	234	1	137.13	6.5	1	137.13	7.75	1	0.1875
GW	235	13	137.13	8.5	1	137.13	26.4	1	0.1875
GW	236	1	137.13	7.75	1	137.13	8.5	1	0.1875
GW	237	1	137.13	7.75	1	138.22	7.75	1	0.06
GW	238	1	137.13	8.5	1	137.82	8.5	1	0.06
GW	239	1	139.47	12.3	1	138.72	12.3	1	0.06
GW	240	1	139.47	9.98	1	138.72	10.14	1	0.06
GW	241	3	139.47	12.3	1	139.47	9.98	1	0.06
GW	242	1	138.72	12.3	1	138.72	10.14	1	0.06
GW	243	3	139.47	9.98	1	138.22	7.75	1	0.06
GW	244	1	138.72	10.14	1	137.82	8.5	1	0.06
GW	245	1	137.82	8.5	1	138.22	7.75	1	0.06
GW	246	1	137.13	7.75	1	136.04	7.75	1	0.06
GW	247	1	137.13	8.5	1	136.44	8.5	1	0.06
GW	248	1	134.79	12.3	1	135.54	12.3	1	0.06
GW	249	1	134.79	9.98	1	135.54	10.14	1	0.06
GW	250	3	134.79	12.3	1	134.79	9.98	1	0.06
GW	251	1	135.54	12.3	1	135.54	10.14	1	0.06
GW	252	3	134.79	9.98	1	136.04	7.75	1	0.06
GW	253	1	135.54	10.14	1	136.44	8.5	1	0.06
GW	254	1	136.44	8.5	1	136.04	7.75	1	0.06
GW	255	1	147.7	-6.5	1	147.7	-7.75	1	0.1875
GW	256	13	147.7	-8.5	1	147.7	-26.4	1	0.1875
GW	257	1	147.7	-7.75	1	147.7	-8.5	1	0.1875
GW	258	1	147.7	-7.75	1	148.79	-7.75	1	0.06
GW	259	1	147.7	-8.5	1	148.39	-8.5	1	0.06
GW	260	1	150.04	-12.3	1	149.29	-12.3	1	0.06
GW	261	1	150.04	-9.98	1	149.29	-10.14	1	0.06
GW	262	3	150.04	-12.3	1	150.04	-9.98	1	0.06
GW	263	1	149.29	-12.3	1	149.29	-10.14	1	0.06
GW	264	3	150.04	-9.98	1	148.79	-7.75	1	0.06
GW	265	1	149.29	-10.14	1	148.39	-8.5	1	0.06
GW	266	1	148.39	-8.5	1	148.79	-7.75	1	0.06
GW	267	1	147.7	-7.75	1	146.61	-7.75	1	0.06
GW	268	1	147.7	-8.5	1	147.01	-8.5	1	0.06
GW	269	1	145.36	-12.3	1	146.11	-12.3	1	0.06
GW	270	1	145.36	-9.98	1	146.11	-10.14	1	0.06
GW	271	3	145.36	-12.3	1	145.36	-9.98	1	0.06
GW	272	1	146.11	-12.3	1	146.11	-10.14	1	0.06
GW	273	3	145.36	-9.98	1	146.61	-7.75	1	0.06
GW	274	1	146.11	-10.14	1	147.01	-8.5	1	0.06
GW	275	1	147.01	-8.5	1	146.61	-7.75	1	0.06
GW	276	1	147.7	6.5	1	147.7	7.75	1	0.1875
GW	277	13	147.7	8.5	1	147.7	26.4	1	0.1875
GW	278	1	147.7	7.75	1	147.7	8.5	1	0.1875
GW	279	1	147.7	7.75	1	148.79	7.75	1	0.06
GW	280	1	147.7	8.5	1	148.39	8.5	1	0.06
GW	281	1	150.04	12.3	1	149.29	12.3	1	0.06
GW	282	1	150.04	9.98	1	149.29	10.14	1	0.06
GW	283	3	150.04	12.3	1	150.04	9.98	1	0.06
GW	284	1	149.29	12.3	1	149.29	10.14	1	0.06
GW	285	3	150.04	9.98	1	148.79	7.75	1	0.06
GW	286	1	149.29	10.14	1	148.39	8.5	1	0.06
GW	287	1	148.39	8.5	1	148.79	7.75	1	0.06
GW	288	1	147.7	7.75	1	146.61	7.75	1	0.06
GW	289	1	147.7	8.5	1	147.01	8.5	1	0.06
GW	290	1	145.36	12.3	1	146.11	12.3	1	0.06
GW	291	1	145.36	9.98	1	146.11	10.14	1	0.06
GW	292	3	145.36	12.3	1	145.36	9.98	1	0.06
GW	293	1	146.11	12.3	1	146.11	10.14	1	0.06
GW	294	3	145.36	9.98	1	146.61	7.75	1	0.06
GW	295	1	146.11	10.14	1	147.01	8.5	1	0.06
GW	296	1	147.01	8.5	1	146.61	7.75	1	0.06
GW	297	3	97	-1.8	0.8	97	1.8	0.8	0.06
GS	0	0	0.0254		' All in in.
GE	0
EK
TL	8	38	9	34	300	0.3556
TL	9	34	10	31	300	0.3556
TL	10	31	11	30	300	0.3556
TL	11	30	12	28	300	0.3556
TL	12	28	13	25	300	0.3556
TL	13	25	14	23	300	0.3556
TL	14	23	15	21	300	0.3556
TL	15	21	16	1	300	0.2032
LD	5	0	0	0	2.5e7	0
EX	0	297	2	0	1	0
GN	-1
FR	0	1	0	0	195	0

300ohm · 2011-09-24, 12:02 AM

Hmm, I removed the Cap Hats off the directors. They are the U shaped pieces on those directors at 2, 3, 4 above. With Cap Hats off for channel 10 and no UHF elements, Raw Gain 10.8 dBi, SWR 1.36, Net Gain 10.7 dBi.

The Cap Hats do seem to play a significant role. But they should be easy enough to cut out from fairly thin sheet aluminum and pop rivet onto the director.

The CM-4251 thread is here :
http://www.digitalhome.ca/forum/show...ight=parabolas

Basically, the only practical thing you can do to it to increase lower channel gain a bit is to increase the bowtie size out to the edge of the small reflector by pop riveting on some sheet aluminum.

kc2cbd · 2011-09-24, 02:46 AM

I did take a closer look at the different pictures and your nec picture.
Your nec file shows 9 larger elements and 9 smaller elements behind where the UHF reflector was.
The stock picture that I found of a CM-3671 shows 10 elements and 8 smaller elements behind where the UHF reflector was.
On my antenna I count 11 larger elements and 9 smaller elements behind where the UHF reflector was.
The missing elements for my antenna are one on the back and one around your reference mark A that are missing from your nec picture.
If you added those elements then the balun connection point would be in the right spot.
I have uploaded another picture that shows most of my antenna.
I was hoping to do this conversion today but I might have to take some measurements and put it back up.
I'm curious if a UVSJ would filter out the UHF signal on that antenna without converting at the present. My hope was to try the 4251 and see if my reception improved on channel 23 (58-1[2]) and if it did I was going to do the conversion and have it done in short order so my system wasn't down for more than an afternoon.
I really do need to have the system back on by this (Saturday) evening.

Of course by just removing the UHF reflector and elements it could increase the VHF and then I could add the directors on later.
I could do that and take all the measurements of the antenna and then put it back up.

I'm curious as to what the wings on those elements from the VHF antenna are for. Could they be some sort of Cap Hat? My antenna doesn't have the same front directors as the 3671 as you can see in the newly uploaded picture to my album.

Thanks 300ohm for your help so far.

300ohm · 2011-09-24, 03:33 AM

Quote:

Of course by just removing the UHF reflector and elements it could increase the VHF and then I could add the directors on later.
I could do that and take all the measurements of the antenna and then put it back up.

Yep, that seems like a wise course of action if you need to get the antenna back up quickly. Take lots of pictures.

For measurements, this is a very good, concise way to do it, except it would be easier for me if the spacing measurements started from the back of the antenna instead of the front.

Quote:

I'm curious if a UVSJ would filter out the UHF signal on that antenna without converting at the present.

Im not quite sure, but I think it may. A HLSJ can filter out FM, so by that logic, a UVSJ may exclude signals above 216 mhz on the VHF port, and exclude signals below 470 mhz on the UHF port.

Quote:

I'm curious as to what the wings on those elements from the VHF antenna are for. Could they be some sort of Cap Hat?

Which elements are you referring to ?

2011-09-12, 12:39 AM	#1
kc2cbd Join Date: Feb 2011 Location: Cookeville, TN, USA Posts: 162	Modify CM-3672 ? I have an old Channel Masters Crossfire CM-3672 I believe. I would like to get some feedback on removing the uhf portion. About 4 or 5 months ago refurbished the antenna and since have acquired a CM-4251 so want to modify the Crossfire for VHF only. My idea was to remove the matching bars and uhf reflectors. I originally thought I was going to be able to remove the whole uhf section but there are vhf elements on that section. Does anyone think removing the uhf elements themselves will hurt the antenna for vhf only? If not I wanted to use those to make a small uhf antenna to point in the opposite direction for a local station that is almost 180 degrees from my Nashville stations. Here is my antenna. There are 2 other photos here https://picasaweb.google.com/1069450...eat=directlink My other question is will running 300ohm twinlead from this antenna to the CM-4251 then from those lugs attach a balun which will then go to my Winegard AP-8780. Will this work or will it hurt reception? I figure that I would be better off getting a UVSJ to combine them. Any comments appreciated.

2011-09-12, 09:44 PM	#3
kc2cbd Join Date: Feb 2011 Location: Cookeville, TN, USA Posts: 162	3672 Mod I don't see any vhf directors on the reflector booms. If you look at my Picasa photo album I linked to you will see that my antenna has matching tubes instead of wires like Brantford_Bill's antenna. Although I'm baffled as to what those bars are for on his UHF folded dipole are. I'm assuming it attaches to another dipole or something. I think if I remove my matching tubes and connect to the VHF dipole connection point instead of the UHF connection point that I connect to right now I think mine will work that way. My other question is will the connection from there to the CM-4251 then subsequently to the pre-amp via a balun. will it work and not degrade the signal of the combining of the 2 antennas?

2011-09-12, 10:55 PM	#8
300ohm Veteran Join Date: Mar 2008 Location: Somewhere in Delaware on the flat side Posts: 7,011	Ideally, what you want is a CM0264 preamp like what I have. (seem to be hard to get now) It has separate 300 ohm inputs for uhf and vhf, and a 75 ohm output, low noise and durable. __________________ My builds/plans (not the latest models) are located here.

2011-09-24, 12:02 AM	#13
300ohm Veteran Join Date: Mar 2008 Location: Somewhere in Delaware on the flat side Posts: 7,011	Hmm, I removed the Cap Hats off the directors. They are the U shaped pieces on those directors at 2, 3, 4 above. With Cap Hats off for channel 10 and no UHF elements, Raw Gain 10.8 dBi, SWR 1.36, Net Gain 10.7 dBi. The Cap Hats do seem to play a significant role. But they should be easy enough to cut out from fairly thin sheet aluminum and pop rivet onto the director. The CM-4251 thread is here : http://www.digitalhome.ca/forum/show...ight=parabolas Basically, the only practical thing you can do to it to increase lower channel gain a bit is to increase the bowtie size out to the edge of the small reflector by pop riveting on some sheet aluminum. __________________ My builds/plans (not the latest models) are located here.

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2011-09-12, 10:22 PM	#5
kc2cbd Join Date: Feb 2011 Location: Cookeville, TN, USA Posts: 162	I don't have any 450ohm ladder available...I do have a few feet of 300 ohm from when I acquired the 4251 and another old really beat up CM antenna which I disassembled. I guess I will see if I can acquire a UVSJ locally, if not I will have to order one and wait for it. I do have a preamp but it isn't in those pictures as I hadn't acquired it yet when I took those pictures. In fact that antenna is now about 10 feet higher than it is in those pictures and has 3 guide wires attached. It is also before I refurbished that antenna too. When I refurbished the antenna and added the pre-amp and raised it up I increased my reception. What I'm trying to achieve is get better reception on channel 23 (virtual 58.1 & 58.2) which doesn't come in reliably right now. My other issue is I need the VHF low and hi cause I recieve 5, 8 & 10 so I need to keep the VHF portion of this antenna. Finding VHF only antennas for 2 to 13 are few and far between and cost more than I'm willing to spend right now. This antenna feeds 2 television and a Hdhomerun reciever for my Mythbox.

2011-09-12, 10:53 PM	#7
kc2cbd Join Date: Feb 2011 Location: Cookeville, TN, USA Posts: 162	I live in Cookeville, TN I can't tell if the dip is worse than my existing antenna. If it is just a little better I would be happy. If it ends up worse than I will make the wife very unhappy as she is happy with it now.

2011-09-12, 11:12 PM	#9
kc2cbd Join Date: Feb 2011 Location: Cookeville, TN, USA Posts: 162	Probably but I got this Winegard AP-8780 and haven't been disappointed. I think I have a local source for the UVSJ that is if Kmart has them at the store. If not I don't know where I can get one locally and will probably order 1 or 2 from SolidSignal which seems to be a good price of less than 2 dollars each.

2011-09-13, 09:00 PM	#10
kc2cbd Join Date: Feb 2011 Location: Cookeville, TN, USA Posts: 162	I added some more pictures to my picasa album that might help make a better assessment. Appreciate your help 300ohm I didn't find a UVSJ locally so I'm going to have to order one...so might order a couple or more considering SolidSignal has them pretty cheap.

2011-09-24, 02:46 AM	#14
kc2cbd Join Date: Feb 2011 Location: Cookeville, TN, USA Posts: 162	I did take a closer look at the different pictures and your nec picture. Your nec file shows 9 larger elements and 9 smaller elements behind where the UHF reflector was. The stock picture that I found of a CM-3671 shows 10 elements and 8 smaller elements behind where the UHF reflector was. On my antenna I count 11 larger elements and 9 smaller elements behind where the UHF reflector was. The missing elements for my antenna are one on the back and one around your reference mark A that are missing from your nec picture. If you added those elements then the balun connection point would be in the right spot. I have uploaded another picture that shows most of my antenna. I was hoping to do this conversion today but I might have to take some measurements and put it back up. I'm curious if a UVSJ would filter out the UHF signal on that antenna without converting at the present. My hope was to try the 4251 and see if my reception improved on channel 23 (58-1[2]) and if it did I was going to do the conversion and have it done in short order so my system wasn't down for more than an afternoon. I really do need to have the system back on by this (Saturday) evening. Of course by just removing the UHF reflector and elements it could increase the VHF and then I could add the directors on later. I could do that and take all the measurements of the antenna and then put it back up. I'm curious as to what the wings on those elements from the VHF antenna are for. Could they be some sort of Cap Hat? My antenna doesn't have the same front directors as the 3671 as you can see in the newly uploaded picture to my album. Thanks 300ohm for your help so far.