Is there anything on UHF out of Barrie now or in the future that would make a CM4221HD necessary for that direction?

About the back-to-back mounting, even with their own reflectors the 2 antennas would affect each other's back door and side lobes. Is it possible to have them at least 25" apart just to be on the safe side?

Seems to have different programing on.

A friend has an extra 4221 Just thought I would put the question out there.
Thanks for the answer. Not a good idea back to back.

I would like to have one point in the general direction of Toronto and one to face the Barrie area.

Why not remove the screen from your 4221 instead. This will result in equal gain from the front and back. It will reduce the forward gain by about 3dB, but combining the two would result in more loss than that.

i read the OTA FAQs and elsewhere when gangning 2 antennas (combining them for 2 different directions), each antenna lead into the combiner should be of exactly equal lengths,


however I read Kens HDTVPrimer write up on merging feedlines, and he says :

When the antennas point in different directions, the cable lengths are not critical in any way,

how can this be?

thanks

In analog it was to avoid ghosting caused by multipath. With some digital tuners multipath can be tolerated to a limit and then that dreaded cliff effect kicks in.

I think the same cable length rule is to eliminate a known risk of having multipath coming from the 2 antennas that could be receiving the same frequency at the same time. Even if you point the antennas in opposing directions, in some cases both antennas may still by chance receive the same channel and deliver these off-set signals to the tuner and that could mess up the tuner's ability to decide which signal information to use.[drop-outs, pixelation]

All in all, IMO, even if your cables are cut exact exact same lengths, any off-set reception of a signal created by a poor splitter/combiner may still create a similar multipath [ghosting] effect.

I read Kens HDTVPrimer write up on merging feedlines, and he says :

When the antennas point in different directions, the cable lengths are not critical in any way,

how can this be?
What he means, when taken fully in context, is that the cable lengths are not critical in any way, other than that the two cables must be identical to each other in length and composition. Otherwise a station received by both antennas will arrive out of phase at the combiner, resulting in unnecessary losses.

But that's actually wrong. The lengths *do* matter, especially for very short interconnections like this. For optimum results, they should be (identical) odd multiples of 1/4 wavelength of the primary/weakest channel of interest. This minimizes losses due to reflections at the connectors, by causing any such reflections to be exactly in phase with the main signal, rather than out-of-phase with it.

But that's only for OCD folks like me to fuss over. ;)

mlord,

ah! yes, it all makes sense that way, thanks,

is the resulting loss still a minimum of 3.5 dbi...


also, is it a better idea to use a coax loop balun for the weakest channel on one antenna, and another coax loop balun for the weakest channel on the other antenna (as opposed to using one loop balun, and one store bought, or 2 store boughts), or does this unsymmetry add to the combiner losses...

based on mlords OCD :eek: 1/4 wave reasoning, for my 122" between 2 baluns, and governing channel #20, i calculate the coax leads to be 64.57318" (includes about 2x3" slack), sound right ?

thanks

...however I read Kens HDTVPrimer...he says :

When the antennas point in different directions, the cable lengths are not critical in any way,

how can this be?

The sum of two signals is a vector addition, i.e. both amplitude and phase combine. For maximum signal, the amplitudes should be maximum (i.e. for identical antennas pointed in the same direction, this means equal or near equal amplitudes) and the phase of the signals must align (i.e. no advance or retardation of the phase).

Thus, the logic for having identical length transmission lines (TL) for identical antennas pointed in the same direction.

Identical length TLs ensure that the amplitude/phase relationship is preserved until the signals are combined.

Any phase advance or delay, or amplitude imbalance, does reduce signal from maximum available.

If the amplitude and phase are slightly away from optimum, it reduces the optimum by relatively small amount.

When antennas point in different directions, or when using two different antennas, the signal interception location(s) are likely to be different in both the vertical and horizontal planes.

This means that the phase and amplitude of the two signals (those antenna outputs) are almost certainly different.

Signals of unspecified different phases and amplitudes are not likely to add in a maximal way regardless of similarities or differences in transmission line lengths (it is a matter of probabilities).

Complete signal cancellation is very unlikely, because this requires a special case of alignment of both amplitude and phase: the amplitudes must be equal and the phase of one signal must be 180 degrees advanced or delayed relative to the second signal. A change away from exactly equal amplitudes and exactly 180 phase difference and the sum of two signals rises quickly from the canceled state.

There are other complications, when two antennas pointed in different directions intercept the same signal; one of the signals may be a reflection (or has taken a different path from the transmitter to the receiving antenna). Reflections or path differences induce phase shifts in the signals as well as amplitude modulations.

There may be special cases, but in general, the cable lengths for two antennas pointing in different directions are not critical. As an example, what length(s) should one choose that would optimize the combination (or for that matter cancellation) of two signals with unknown amplitude and phase relationships?

In these situations, uncontrolled variables undermine the common objectives of selecting specific cable lengths.

thanks tripelo,

very informative explanation...


As an example, what length(s) should one choose that would optimize the combination (or for that matter cancellation) of two signals with unknown amplitude and phase relationships?


i always wondered about that.

mlord,

ah! yes, it all makes sense that way, thanks,

is the resulting loss still a minimum of 3.5 dbi...
Huh? Loss? Huh?

The idea of combining two antennas, is to point them exactly in the same direction, for GAIN, not loss. In this situation, *everything* should be identical: antennas, baluns, coax leads, tilt, and direction.

If you're pointing them differently, then goodness knows what the result will be for any given wavelength (channel).

Some vectors will be stronger (gain), others will be weaker (loss).

I cannot help you with that -- the sensible thing is to use coax switches or separate tuners, and NOT combine them when pointed differently.

Cheers

mlord,

thanks, that ultimately is what ill probably do, its just that several members (maybe many) have succeeded in combining antennas in different directions, some not even identical, but i suspect they were in a specific ideal situation to achieve this successfully :eek:

Hi guys,
just picked up an VHF antenna and this spring (non slippery, warmer weather) I am going to add it to my existing setup. I have a DB8 and a Channel Master 7778 preamp so I will be using this as my joiner.
I could only find one response in a search regarding the spacing of a VHF and UHF antenna regarding installation. It stated that they recommend a minimum of 2 feet and the more the better.
Also is it like in ganging or stacking antennas? What I mean is do the coax feeds from each antenna need to be the same length going to the preamp?
Thanks again
Mark

Antennas "should" be located to minimize gain pattern perturbations due to nearby objects.
The UHF antenna should be located several wavelengths (2-5 feet) away from other objects.
Hi-VHF wavelengths are longer, hence antenna should be located about 3 times further away.
Lo-VHF wavelengths are longer than Hi-VHF by another (impossible) factor of 2-3.
So you gotta work with what you got.....on a common rotor, most are only a couple feet away,
which means they are for sure interacting.....if not on a common rotor, drop the VHF antenna
down a few more feet.

VHF and UHF cable lengths do NOT have to be the same length since they do not interact in the Preamp.

It stated that they recommend a minimum of 2 feet and the more the better.Hi Mark, I have read many times of people using a two antenna setup have them separated by at least a full wavelength of the lowest UHF channel (14). More spacing may be needed since you are going for Global CH 6 in the low band. As we all know here, YMMV for the best spacing at your location. Best to experiment plenty if you can to find the most optimal result.

Also, CFPL 10 is extremely strong at your location in TVfool. A real vhf antenna that close in might overload the CM7778 too far into the red. Passing the vhf signals instead of amplifying them would greatly help in that case. Looking forward to pictures of this once it is up and running!

edit: holl_ands posed first by seconds

what if I put an attenuator right at the VHF input of the preamp. Lets say 8 db.
Would this tame 10 CFPL?
Aw you know me. When the time comes I will mess with the set up at the least a few times till I find the right set up.
Indeed I am working with a rotor. The pole is around 4 feet from the rotor so I have a limited space to work with.

You may have already thought of this, when you attenuate for VHF/Ch.10, you might want to notice/be careful how this impacts Erie 12.x.

Hi Mark,

Another channel you may need to watch out for is FM96. 190 kW radio station may cause some problems.
Also CBCf has been assigned a ERP of 10kW station on rf 7(modied pattern)
CFPL has been assigned a ERP of 10kW (full omni directional pattern) when it goes digital.

Note: that an 8 bay bowtie does have gain on VHFhi Channels. And has a negative gain on FM & VHFlo channels.

I would suggest a attunetor that negates the total gain of the pre amp. Since, CFPL is a very strong signal. It shouldnt matter if you attunate the signal 8 or 20 Db.

Read ya l8r,
Al

This is yet another reason that I went with a Yagi UHF: I can achieve greater antenna separation.

Another channel you may need to watch out for is FM96. 190 kW radio station may cause some problems.
Also CBCf has been assigned a ERP of 10kW station on rf 7(modied pattern)
CFPL has been assigned a ERP of 10kW (full omni directional pattern) when it goes digital.

Note: that an 8 bay bowtie does have gain on VHFhi Channels. And has a negative gain on FM & VHFlo channels.

I would suggest a attunetor that negates the total gain of the pre amp. Since, CFPL is a very strong signal. It shouldnt matter if you attunate the signal 8 or 20 Db.

Read ya l8r,
Al
There is an FM trap built into the preamp. Further, most Delhi VHF antennas come with an option to attenuate FM or to enable reception by breaking off an element.

Note that VHF reception on the 8-bay will be blocked by the pre-amp when it is set to separate inputs.

Thank you all for your input's. They will all be heeded. My biggest concern was the spacing and coax length's.

There is an FM trap built into the preamp. Further, most Delhi VHF antennas come with an option to attenuate FM or to enable reception by breaking off an element.

Indeed you are correct regarding this matter. I will not be breaking the notched portion on the antenna to permit FM reception.

I am trying to grasp the whole preamp setup (combined or separate feature). At this current time I have the combined on (default) and FM trap enabled. The VHF portion has a termination and weather proof boot installed.
Now when I add the VHF antenna and put it to the VHF portion on the preamp do I have the choice to not amplify the VHF portion of am I stuck with the 3.0 NF for VHF and 16 dB amplification. What does combined do? Is it better to set it to separate?

@Jase, yes an UHF yagi does has its advantages but I do love my bowtie.

@goforit I am wondering if I will be able to pull 12-X out of Erie, Pa. after this is set up.