Original Posted By: owner-pietenpol-list-server(at)matronics.com
Larry,You can't combine antennas when broadcasting! When you broadcast on thehandheld, part of the energy goes right into the ELT's amplifier, heating itup. Likewise, if the ELT is broadcasting, it is toasting your handheld!Heat is the bane of all electronic equipment. Electronics will continue towork unless you let the smoke out. Using a T in the antenna line can letthe smoke out.Yipes!The following is quoted from page 148 of Avionics Volume 1 Every Pilot'sGuide to Aviation Electronics by J. M. Ferrara:"Multiple Use of Antnennas: How about two transmitters operating from oneantenna? This is more difficult, as we cannot simply connect bothtransmitters to the antenna. If we did, some of the power from thetransmitter in use would go to the unused transmitter and be wasted in theform of heat, in addition to the second transmission line disruption of thecharacteristics of the antenna system; so that the proper load would notappear at either transmitter. In order to use the one antenna for twotrnasmitters or two transievers, we would have to have a relay that switchedthe antenna from one transmitter to the other when the microphone selectorswitch was placed on the desired transmitter."Disscusion on page 142 of the same book indicates what happens if the properantenna load should not appear at the transmitter. When you read this, theword "load" and "antenna" mean the same thing:"Let's assume that the load is not perfect. The transmitter will not seethe proper load so that it will not produce the maximum power the antenna iscapable of. In addition, the power that is produced will not all be used bythe load. This means that if we have a poor load (bad antenna), some of thepower generated by the transmitter will be reflected back from the load tothe transmitter, and only some of the power will be used by the load."Imagine that the transmitter sends a wave out the antenna lead to the tip ofthe antenna. Ideally, the energy from that wave goes out into space and theantenna is 100% efficient. Imagine another case where the wave goes out butnone of the energy in that wave is radiated from the antenna. In this casethe energy is "reflected" back to the transmitter and all the power that wassent out comes back into the transmitter. The real world exists somewherebetween these two cases but we always strive to achieve the first case. Theefficiency of an antenna system is measured by the Standing Wave Ratio orSWR. It is the measure of the amount of energy transmitted to the antenna(the forward power) compared to the amount of energy relected back to thetransmitter (the reflected power). If the forward equals the reflected, theSWR is infinite. If the antenna is 100% efficient, i.e. no energy isreflected back, then the SWR is 1. The worst you want to see with your radiois about 6 and this is waht a bent whip with a minimum ground plane willgive you. With the 5 watts or so you put out and an SWR of 6, about 2.5watts are being reflected back. Adding the T is only going to make thisworse. Transmitters are not designed to take more than a certain amount ofrelected energy. Any energy that is reflected back to the transmitter mustbe absorbed by the hardware and this is in the from of heat. Heat willdestroy the transmitter. It might not happen all at once but one day,either your ELT or the the handheld will not want to work and you will bescratching your head.Using a T will cause some of the energy from your handheld to go right intothe ELT, cooking it. In addition, since the load is not correct for thehandheld, it will suffer excessive reflection.I would advise against the T. You need to go with separate antennas. Thatis why airplanes have so many. Combining receiving antennas like two vorreceivers using one NAV antenna is ok if hooked up with a splitter.chris bobka-----Original Message-----