The SNA600 will operate well below 500 MHz. It is marked 500MHz because we didn't make receivers below 500 MHz until the great digital changeover of all the TV stations in 2009. The SNA-600 measures lower than a 2:1 SWR (Standing Wave Ratio) from 440 MHz to 600 MHz when the antenna arms are fully extended. Less than 2:1 SWR is considered a standard antenna range measurement. The marked range for fully extended is only down to 500 MHz but the antenna will operate much lower than that.

The new stainless steel SMA female connector is pressed into the aluminum front panel and is an interference fit. Any possible panel to barrel gaps are filled by us with a Loctite gap filler. The antenna wire itself has an O-ring in the nut assembly that seals the wire antenna to the nut of the male connector so any water running down the antenna cannot enter the connector. Then the only possible water entry is if the SR is upside down or angled down and water runs into the inverted nut, around the threads, into gaps between the center insulator and the barrel and down into the unit. You can seal against this improbable occurrence with a small drop (dab) of Vaseline or other petroleum jelly inside the SMA connector applied right on the white insulator. The Vaseline will prevent leakage by this path totally, even if the unit is dropped into water and if the SR (and attached camera) are submerged, you've got bigger problems anyway. The Vaseline does not affect the RF at all. If the Vaseline gets dusty when the SMA is removed, just clean it with a cloth or Q-tip and some clean Vaseline.

This was a posting to RAMPS about "Calculating Intermod Frequencies".

Here's a chance for a 40 page dissertation that I'm not going to take. In the interest of keeping it simple and easy to remember, I'll make some general statements that are 99% true, i.e., errors are 40 dB down and good enough for sound mixers. ;-)

Intermod is calculated in exactly the same way by all the programs and has little or nothing to do with i.f. frequencies.

Odd order intermod (3rd, 5th, etc.) is much more of a problem than even order intermod (2nd, 4th, etc.) because with odd orders it is possible to generate interference that is very close to the receiver frequency. This interference can therefore pass straight through the receiver front end filters. Of the odd orders, third order intermod is of greatest concern because the interference is always at a much higher level than 5th or 7th.

Second and 4th order are of lesser concern because the carriers that generate them cannot be close to the receiver frequency and will be filtered out by the front end. (Our IFB receiver is one of the few receivers for which this statement is not true. The IF is so low, 70 kHz (!!), that the image at 140 kHz from the operating frequency can be generated by 2nd order intermod.)

Intermod due to wireless transmitters getting into the receiver is not a factor if the transmitters are all, repeat ALL, 20 feet or more away from a good quality receiver. Intermod between a transmitter and a TV station does not follow the 20 foot rule for the TV station component obviously, or for two TV stations.

Intermod generation between transmitters is more of a problem in most situations particularly if the transmitters have standard output stages and are closer than 5 feet apart. The intermod frequencies are exactly the same as in receivers and any intermod program will catch them. I have seen fairly strong 5th and 7th order when transmitters are only a few feet apart.

A quick discussion of image frequencies is appropriate here since most programs also calculate images and this is where different brands do have differences. The knowledge of the i.f. frequency is critical in determining where the image frequencies will lie. A first low i.f frequency says the image frequency will be a small distance from the receiver operating frequency. Most modern receivers have a high first i.f. such as 244 MHz. This puts the image at 2 x i.f = 488 MHz away from the tuned frequency. Almost 500 MHz away means the front end filters can strongly suppress it. Generally, with modern i.f.'s, other wireless transmitters don't cause image problems. It's from other high power transmitters in the environment.

If the above is the Reader's Digest version of intermod stuff, here's the Cliff's notes:

1. Third order intermod is the biggie.
2. Any program will calculate third order.
3. Third and higher order between transmitters is usually the main problem.
4. You only need to worry about intermod if the transmitters are close to the receivers (less than 20 feet) or each other (less than 5 feet) or in other words, always.

Since the original question was "Mathematical formula in finding Intermodulation free frequencies" here it is:
1.Find the difference between two transmitter frequencies. 
2.Subtract the difference from the lower frequency and add it to the higher frequency. Those will be the two interfering frequencies, so don't have a receiver on those frequencies.

Example: Transmitter A=525.000 MHz, transmitter B=550 MHz. Difference is 25 MHz. Don't have a system on 500.000 (525-25MHz) or on 575.000 (550+25MHz). Note that you can't screw up the math. Doing it wrong such as 525 +25 and 550-25 gives you the original starting frequencies. Also note that two systems don't have intermod problems. It takes two transmitters to generate garbage on a third frequency and you have to have a third system in order to have something to interfere with. TV stations do count as a transmitter, however, if the signal is strong at the receivers, i.e., equivalent to a wireless transmitter at 20 feet or less.

Well at least I managed to keep it to only one full page (if you use a small font).

The best way is to use three 2-way splitters on 3 of the outputs from your 4 way multicoupler and run the last output as usual for a total of 7 outputs. You will have a 3 dB+ loss on the split lines. If you are working at relatively close distances (under 100 feet) and/or in a RF noisy environment, you will not notice any change in performance. The reason is that if you are close you have plenty of signal and a small loss won't be noticeable. If you are in an RF noisy environment, the splitter will drop the signal and the noise both by 3 dB and the signal to noise ratio at the input remain the same. This is true until the attenuated external noise is less than the front end input noise of the receiver. I would try the passive splitters and save the money. Plus, splitters are always handy things to have around even if you eventually get a UMC16B.

Yep, we do. We've had so many customers having problems when using other brand wide band RF amplifiers in their antenna systems that I caved in and set up some wide band versions of our distribution amps and amplifiers. They have a 230 MHz bandwidth and cover all our standard blocks. At least they still have a very high intercept point with low noise figures. The prices are the same as the narrower, 2 block wide units. The web site may not be changed yet but the sales crew have the details. The wide band version of the UHFM-50 is the UHFM-230. The UMC16A is the 50 MHz version and the UMC16B is the 230 MHz wide band version.

Score the insulation of a 50 Ohm coax such as RG58 (RG174 is OK if it is less than 5 feet) one quarter wavelength from the end of the cable. Do it very gently and don't score or even scratch the stranded shield wires since they will easily break off. Remove the outer insulation. Pull the shield down so that it has some slack in it with a bulge near the remaining outer insulation of the coax. Poke an opening in the shield with a blunt tool near the remaining insulation. The hole should large enough to pull the center conductor with the inner insulation on it through the opening. Straighten the shield so that you have a quarter wavelength shield wire and a quarter wavelength center conductor. These are the two arms of your dipole. Fold the shield back along the outer insulation of the coax cable. The center conductor is left pointing in its original direction. Now cover the center conductor and the shield with a half wavelength or more of shrink tubing to make it look nice. This antenna works best if the center conductor and the folded back shield are both in the open air since both the folded back shield and the center conductor radiate.

The color of the antenna caps or bands correspond to the last digit of the block number of the system. The universal color code used for resistors was picked for the code.

Here's what I think I know about antenna orientation. If you are outdoors with the transmitter in the normal position with the transmitter antenna vertical, then you will get the best antenna strength ON THE AVERAGE with the dipole antenna arms also vertical. Inside where you have lots of reflections and the antenna signals are coming from all over, then vertical or horizontal polarizations are about the same. So vertical polarization is still, the safest orientation. There are two exceptions to this. If the transmitter antenna is horizontal, then the dipole arms should also be horizontal. Or if you can't separate the two dipole antennas by ideally at least a 1/2 wavelength (8 inches) to get good space diversity, then you can go to polarization diversity by rotating one antenna 90 degrees to the other. My advice is, if you can get the antennas 8 inches apart or more, then go with the vertical polarization.

The Venue receiver will do both antenna and frequency diversity simultaneously. The reduction in drop outs and noise ups should be just as dramatic as the reduction going from single antenna reception to regular diversity reception. Definitely use two antennas. An antenna port is a terrible thing to waste.

Usually it won't. Our receivers' front ends are very quiet and within a few dB of the theoretical limits for noise. The range limitations are usually due to other interfering noise sources in the environment. Additional gain before the receiver will not increase range but will lead to increased RF intermodulation and RF overload. The only time you want gain before a well designed receiver is to neutralize cable and/or splitter losses. In this case, the gain must go before the cable or splitter loss and it must be a high quality amplifier such as our IFM50. Putting the gain after the loss is too late since the signal to noise ratio is now poor and gain won't improve it. An analogy for RF is the same as for music; lots of amplification can't overcome poor source material.