a better understanding of the present invention is obtained when the following non - limiting detailed description is considered in conjunction with following figures . as was previously explained , it is necessary to calibrate the amplitude and phase errors between the radio channels due to differences in the rf devices resulting from tolerances and operating frequencies in the antenna array . since these errors tend to change over time e . g . due to temperature variations , real - time dynamic calibration is essential . the commonly used methods today to cope with this need are the use of an external calibration unit as shown in fig1 and the use of a self - calibration mechanism as demonstrated in fig2 . as may be seen in fig1 , the major disadvantage of the external calibration scheme is that it requires additional radio unit with tx / rx calibration channels and additional control mechanisms to control and synchronize the calibration transmissions together with the regular modem transmissions . in addition , it requires also that the external circuitry would link the tx / rx channels with the calibration unit ( e . g . by incorporating couplers and combiners into the antenna array design ). the example of a self - calibration scheme as demonstrated in fig2 is based on the use of one or more of the tx / rx channels of the operative antennas for the purpose of self - calibration . specifically , during the tx calibration , all the tx channels transmit the calibration signal while one or more of the rx channels are operative to receive it . after that , at rx stage of the calibration , all the rx channels receive while one or more of the tx channels transmit the calibration signal . the disadvantage of this method is that it uses a lot of additional components for the calibration , such as the switches to route the signal , couplers and combiners . this method requires special radio hw design and cannot be used on the basis of the existing design . fig3 is an example demonstrating an embodiment of the invention for receiving a signal ( a second signal ) that has been derived from the transmission of another ( first ) signal by utilizing the reflecting effect , in this case the non - ideality of the antenna which causes that effect . the apparatus ( 300 ) illustrated in this fig3 comprises transmitter ( 310 ), receiver ( 320 ), tx / rx combining / separation block ( 330 ), e . g . a circulator , antenna ( 340 ) and two reflected signals . the first reflected signal ( 350 ) is derived from the tx / rx combining / separation block leakage due to the fact that when the calibration signal is transmitted it passes through tx / rx combining / separation block ( 330 ) but part of it finds its way back to receiver ( 320 ). the second reflected signal ( 360 ) is derived from the antenna &# 39 ; s return loss ( instead of being transmitted over the air ). system ( 400 ) illustrated in fig4 comprises four radio heads 410 , 420 , 430 and 440 . in the system exemplified , radio head 410 is selected as an anchor radio head . during rx calibration , anchor radio head ( 410 ) transmits the calibration signal to four radio heads ( i . e . including to itself ). thus , the signal is conveyed via all four rx paths while the receivers are set to a normal operational mode . for each of the four receivers , a processor is adapted to obtain at least one difference in the respective rf device resulting from tolerances and operating frequencies in the antenna array and determine how that respective rf device should be calibrated to compensate for the differences detected . in system 500 illustrated in fig5 there are four radio heads 510 , 520 , 530 and 540 . in this example , radio head 510 is selected as the anchor radio head . during the tx calibration all radio heads ( 510 , 520 , 530 and 540 ) transmit the calibration signals towards the anchor radio head ( 510 ) either one at a time or simultaneously . the signals which can be the same signal for all radio heads or different ones ( at least for some of the radio heads ) are conveyed along all four tx paths while the respective channels are set to a normal operational mode . as will be appreciated by those skilled in the art , the preferred calibration scheme will be a combination of both the rx calibration and the tx calibration . in system 600 illustrated in fig6 there are four radio heads 610 , 620 , 630 and 640 . each of the radio heads comprises four signal rf port 612 , 622 , 632 and 642 respectively , and four antennas 616 , 626 , 636 and 646 , respectively . the two arrows ( 650 , 660 ) indicate two signal transmissions options . the option of internal transmission ( 650 ), is carried out by transmitting the signal via cables ( e . g . within the network ), whereas the option of external transmission ( 660 ) is materialized through the antennas and via the air . although that from a practical point of view the system of the present invention is independent of the path along which the signal is transmitted , still according to an embodiment of the invention system ( 600 ) is optionally capable of determining automatically which path would be better to transmit the calibration signal , e . g . depending on the isolation between the various antennas and the anchor antenna , and to transmit it accordingly . such an automatic determination may be carried out as explained above either by having the calibration algorithm choosing if the wire path should be added to the air path , or by choosing an anchor antenna so that the signal transmitted from / to the anchor antenna never arrives to another antenna by wire - air “ multi - path ”. fig7 and 8 illustrate a tx phase and an rx phase , respectively , of calibration in an antenna array that comprises 4 units / antennas . the time period used for calibrating antenna array according to the present invention is only a short period / slot in a calibration frame . in these figs ., the transmit / receive paths include gain control circuitry , capable of changing its state dynamically while entering the calibration phase ( calibration zone ) and while exiting same , according to a pre - defined calibration algorithm . the current state of the gain control circuitry is derived based on losses incurred along the path connecting the various elements in the respective transmit / receive chain ( s ). this self calibration scheme is divided into two phases . the first phase , rx calibration phase , utilizes one of the radio heads ( the anchor radio head ) for transmitting while the rest of the radio heads are receiving . in the next phase , tx calibration phase , utilizes the same anchor radio head for receiving while transmitting is done from the rest of the radio heads . for k radio heads there are to handle 2 *( k − 1 ) measurements . specifically , for two radio heads ( k = 1 , 2 ), there are 2 measurements . in this case the scheme is indifferent to choosing the anchor radio head and each of the two measurements may either be considered as the rx calibration phase or the tx calibration phase . from the above measurements , factored ratio between the rx and tx paths for all the radio heads can be calculated : c kl = c lk — reciprocal frequency response between transmit / receive path k and receive / transmit path 1 . for four radio heads ( k = 1 , 2 , 3 , 4 ), there are 2 phases of 3 measurements each total of 6 measurements over three different pairs . assume radio head # 1 is the anchor radio head : h 12 = tx 1 rx 2 c 12 h 13 = tx 1 rx 3 c 13 h 14 = tx 1 rx 4 c 14 h 21 = tx 2 rx 1 c 21 h 31 = tx 3 rx 1 c 31 h 41 = tx 4 rx 1 c 41 from the above measurements , a factored ratio between the rx and tx paths for all the radio heads can be calculated : here , the step of selecting first antenna from among the plurality of antennas can be based upon evaluating the set of measured pairs ( 1 2 , 1 3 and 1 4 in this example ) resulting with best quality in terms of , for example , signal to noise and interference ratio ( sinr ) at receiving antenna port . thus , one can avoid measuring a pair which results with low quality . for example , one can choose radio head # 2 as the anchor radio head resulting the measured pairs to be ( 2 1 , 2 3 and 2 4 ) thus avoiding 15 measuring the ( 1 3 , 1 4 and 3 4 ) pairs . in this scheme , four radio heads are taken as an example , enabling another level of freedom in avoiding measurements of specific pairs while using two anchor radio heads . the first phase , rx calibration , utilizes one of the anchor radio heads (# 1 in this example ) for transmitting while two other non - anchor (# 3 and # 4 in this example ) radio heads are receiving . also as part of the rx calibration phase , the other anchor radio head (# 2 in this example ) is used for transmitting while only one of the non - anchored (# 3 in this example ) radio heads is receiving . the second phase , tx calibration , utilizes one of the anchor radio heads (# 1 in this example ) for receiving while two other non - anchor (# 3 and # 4 in this example ) radio heads are transmitting . also as part of the rx calibration phase , the other anchor radio head (# 2 in this example ) is used for receiving while only one of the non - anchor (# 3 in this example ) radio heads is transmitting . for k radio heads there are 2 *( k − 1 ) measurements . in this example , 1 2 and 3 4 measurements are avoided . h 13 = tx 1 r 3 c 13 h 14 = tx 1 rx 4 c 14 h 23 tx 2 rx 3 c 23 h 31 = tx 3 rx 1 c 31 h 41 = tx 4 rx 1 c 41 h 32 = tx 3 rx 2 c 32 from the above measurements , a factored ratio between the rx and tx paths for all the radio heads can be calculated : it is to be understood that the above description only includes some embodiments of the invention and serves for its illustration . numerous other ways of carrying out self testing and / or calibration of antennas in an antenna array in wireless telecommunication networks may be devised by a person skilled in the art without departing from the scope of the invention , and are thus encompassed by the present invention . for example , it should be clear to any person skilled in the art that the signal transmitted from the anchor antenna towards the other antennas in the array could be the same signal as it transmits for its self - calibration , or can be a different signal . similarly , the pre - defined signals transmitted by each of the antennas while calibrating the tx channels could be the same signals or different signals . also , shifting the functionality from one device to another device within the systems described is encompassed by the present invention . the present invention has been described using non - limiting detailed descriptions of preferred embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention . it should be understood that features described with respect to one embodiment may be used with other embodiments . variations of embodiments described will occur to persons of the art . furthermore , the terms “ comprise ”, “ include ”, “ have ” and their conjugates shall mean , when used in the claims “ including but not necessarily limited to ”. also when term was used in the singular form it should be understood to encompass its plural form and vice versa , as the case may be .